addr-spec = local-part "@" domain

Mailbox = Local-part "@" ( Domain / address-literal )

addr-spec = local-part "@" domain

Mailbox = Local-part "@" ( Domain / address-literal )

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

addr-spec = local-part "@" domain

Mailbox = Local-part "@" ( Domain / address-literal )

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Domain = sub-domain *("." sub-domain)

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Domain = sub-domain *("." sub-domain)

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Domain = sub-domain *("." sub-domain)

addr-spec = local-part "@" domain

Mailbox = Local-part "@" ( Domain / address-literal )

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

Errata ID 1081, reported 2007-11-20, identifies a problem with the evolution of naming of top-level domains and the text of RFC 1123. It reads: Section 2.1 says: However, a valid host name can never have the dotted-decimal form #.#.#.#, since at least the highest-level component label will be alphabetic. It should say: However, a valid host name can never have the dotted-decimal form #.#.#.#, since at least the highest-level component label will be not all-numeric. Notes: RFC 3696 section 2 states: "There is an additional rule that essentially requires that top-level domain names not be all-numeric." The eleven IDN test TLDs created in September 2007 contain hyphen-minus as specified in the IDNA RFCs. It should say: However, a valid host name can never have the dotted-decimal form #.#.#.#, since this change does not permit the highest-level component label to start with a digit even if it is not all-numeric. Notes: This is a correct identification of the problem, but the wrong fix. RFC 3696, which ID 1081 cites, is an informational document that is deliberately relaxed about the fine details and says so. It is not relevant to determination of the text that should have been (with perfect knowledge of the future) in 1123. Based on discussions when we were doing RFC 1591 and subsequently, the expectation then (and presumably when 1123 was written) was that the name of any new TLD would follow the rules for the existing ones, i.e., that they would be exactly two or three characters long and be all-alphabetic (which is exactly what 1123 says). The slightly-odd "will be" language in 1123 was, I believe, because that restriction was expected to be enforced by IANA, rather than being a protocol issue. ICANN, with a different set of assignment policies, effectively eliminated the length rule with the TLDs allocated in 2000. IDNA (RFC 3490) uses a syntax for IDNs that requires embedded hyphens in TLDs if there were ever to be an actual IDN TLD (hence the comment in ID 1081 about the IANA IDN testbed). While the proposed correction in Errata ID 1081 would fix the problem by imposing the narrowest possible restriction ("not all-numeric"), the original host name rule and the original statement in 1123 both assume the possibility of a minimal check to differentiate between domain names and IP addresses, i.e., checking the first digit only. Because I believe that there are probably implementations that depend on such minimal parsing --some probably ancient and embedded-- it would appear to be wise to relax the rule as little as possible and, in particular, to restrict the "leading digit" exception to domains below the top-level, as 1123 effectively does. The suggested text above reflects that reasoning. Because of the possible consequences of this issue, I would hope that it would be discussed with the relevant DNS-related WGs, the Root Server Advisory Committee (RSAC), and with IANA for comment and as a heads-up. This issue is substantive enough that it should probably be dealt with by a document that explicitly updates 1123 and that is processed on the Standards Track, but an accurate statement in the errata is the next-best option until that can be done. In the interim and while this suggestion is being discussed, Errata ID 1081 should probably be taken out of "validated" status.

Errata ID 1081, reported 2007-11-20, identifies a problem with the evolution of naming of top-level domains and the text of RFC 1123. It reads: Section 2.1 says: However, a valid host name can never have the dotted-decimal form #.#.#.#, since at least the highest-level component label will be alphabetic. It should say: However, a valid host name can never have the dotted-decimal form #.#.#.#, since at least the highest-level component label will be not all-numeric. Notes: RFC 3696 section 2 states: "There is an additional rule that essentially requires that top-level domain names not be all-numeric." The eleven IDN test TLDs created in September 2007 contain hyphen-minus as specified in the IDNA RFCs. It should say: However, a valid host name can never have the dotted-decimal form #.#.#.#, since this change does not permit the highest-level component label to start with a digit even if it is not all-numeric. Notes: This is a correct identification of the problem, but the wrong fix. RFC 3696, which ID 1081 cites, is an informational document that is deliberately relaxed about the fine details and says so. It is not relevant to determination of the text that should have been (with perfect knowledge of the future) in 1123. Based on discussions when we were doing RFC 1591 and subsequently, the expectation then (and presumably when 1123 was written) was that the name of any new TLD would follow the rules for the existing ones, i.e., that they would be exactly two or three characters long and be all-alphabetic (which is exactly what 1123 says). The slightly-odd "will be" language in 1123 was, I believe, because that restriction was expected to be enforced by IANA, rather than being a protocol issue. ICANN, with a different set of assignment policies, effectively eliminated the length rule with the TLDs allocated in 2000. IDNA (RFC 3490) uses a syntax for IDNs that requires embedded hyphens in TLDs if there were ever to be an actual IDN TLD (hence the comment in ID 1081 about the IANA IDN testbed). While the proposed correction in Errata ID 1081 would fix the problem by imposing the narrowest possible restriction ("not all-numeric"), the original host name rule and the original statement in 1123 both assume the possibility of a minimal check to differentiate between domain names and IP addresses, i.e., checking the first digit only. Because I believe that there are probably implementations that depend on such minimal parsing --some probably ancient and embedded-- it would appear to be wise to relax the rule as little as possible and, in particular, to restrict the "leading digit" exception to domains below the top-level, as 1123 effectively does. The suggested text above reflects that reasoning. Because of the possible consequences of this issue, I would hope that it would be discussed with the relevant DNS-related WGs, the Root Server Advisory Committee (RSAC), and with IANA for comment and as a heads-up. This issue is substantive enough that it should probably be dealt with by a document that explicitly updates 1123 and that is processed on the Standards Track, but an accurate statement in the errata is the next-best option until that can be done. In the interim and while this suggestion is being discussed, Errata ID 1081 should probably be taken out of "validated" status.

The maximum total length of a user name or other local-part is 64 octets.

labels 63 octets or less

Domain = sub-domain *("." sub-domain) Let-dig = ALPHA / DIGIT Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig

Domain = sub-domain *("." sub-domain) Let-dig = ALPHA / DIGIT Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Domain = sub-domain *("." sub-domain)

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Domain = sub-domain *("." sub-domain)

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Domain = sub-domain *("." sub-domain)

However, there is a restriction in RFC 2821 on the length of an address in MAIL and RCPT commands of 254 characters. Since addresses that do not fit in those fields are not normally useful, the upper limit on address lengths should normally be considered to be 254.

However, there is a restriction in RFC 2821 on the length of an address in MAIL and RCPT commands of 254 characters. Since addresses that do not fit in those fields are not normally useful, the upper limit on address lengths should normally be considered to be 254.

The maximum total length of a domain name or number is 255 octets.

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS] quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS]

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS] quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS]

qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext

obs-qp = "\" (%d0 / obs-NO-WS-CTL / LF / CR) obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

The maximum total length of a user name or other local-part is 64 octets.

The maximum total length of a user name or other local-part is 64 octets.

address-literal = "[" ( IPv4-address-literal / IPv6-address-literal / General-address-literal ) "]"

IPv4-address-literal = Snum 3("." Snum) Snum = 1*3DIGIT ; representing a decimal integer ; value in the range 0 through 255

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]

domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]

domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]

domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

address-literal = "[" ( IPv4-address-literal / IPv6-address-literal / General-address-literal ) "]"

IPv4-address-literal = Snum 3("." Snum) Snum = 1*3DIGIT ; representing a decimal integer ; value in the range 0 through 255

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

address-literal = "[" ( IPv4-address-literal / IPv6-address-literal / General-address-literal ) "]"

IPv4-address-literal = Snum 3("." Snum) Snum = 1*3DIGIT ; representing a decimal integer ; value in the range 0 through 255

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

address-literal = "[" ( IPv4-address-literal / IPv6-address-literal / General-address-literal ) "]"

IPv4-address-literal = Snum 3("." Snum) Snum = 1*3DIGIT ; representing a decimal integer ; value in the range 0 through 255

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

address-literal = "[" ( IPv4-address-literal / IPv6-address-literal / General-address-literal ) "]"

IPv4-address-literal = Snum 3("." Snum) Snum = 1*3DIGIT ; representing a decimal integer ; value in the range 0 through 255

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

address-literal = "[" ( IPv4-address-literal / IPv6-address-literal / General-address-literal ) "]"

IPv4-address-literal = Snum 3("." Snum) Snum = 1*3DIGIT ; representing a decimal integer ; value in the range 0 through 255

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

address-literal = "[" ( IPv4-address-literal / IPv6-address-literal / General-address-literal ) "]"

IPv4-address-literal = Snum 3("." Snum) Snum = 1*3DIGIT ; representing a decimal integer ; value in the range 0 through 255

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

Comments and folding white space SHOULD NOT be used around the "@" in the addr-spec.

4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.

Comments and folding white space SHOULD NOT be used around the "@" in the addr-spec.

4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.

obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS] quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS]

obs-domain = atom *("." atom) atom = [CFWS] 1*atext [CFWS]

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS] quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS]

obs-domain = atom *("." atom) atom = [CFWS] 1*atext [CFWS]

dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Comments and folding white space SHOULD NOT be used around the "@" in the addr-spec.

4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Comments and folding white space SHOULD NOT be used around the "@" in the addr-spec.

4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.

dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Comments and folding white space SHOULD NOT be used around the "@" in the addr-spec.

4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.

dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

Domain = sub-domain *("." sub-domain) Let-dig = ALPHA / DIGIT Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]

quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] qcontent = qtext / quoted-pair qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext quoted-pair = ("\" (VCHAR / WSP)) / obs-qp

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Domain = sub-domain *("." sub-domain)

quoted-pair = ("\" (VCHAR / WSP)) / obs-qp VCHAR = %d33-126 ; visible (printing) characters WSP = SP / HTAB ; white space

domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

dtext = %d33-90 / ; Printable US-ASCII %d94-126 / ; characters not including obs-dtext ; "[", "]", or "\"

obs-dtext = obs-NO-WS-CTL / quoted-pair obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

obs-dtext = obs-NO-WS-CTL / quoted-pair obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

obs-dtext = obs-NO-WS-CTL / quoted-pair obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

Domain = sub-domain *("." sub-domain)

quoted-pair = ("\" (VCHAR / WSP)) / obs-qp VCHAR = %d33-126 ; visible (printing) characters WSP = SP / HTAB ; white space

domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]

domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

obs-qtext = obs-NO-WS-CTL obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

obs-qp = "\" (%d0 / obs-NO-WS-CTL / LF / CR) obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

obs-qtext = obs-NO-WS-CTL obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CRLF = %d13.10

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CRLF = %d13.10

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CRLF = %d13.10

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CRLF = %d13.10

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CRLF = %d13.10

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext

obs-qp = "\" (%d0 / obs-NO-WS-CTL / LF / CR) obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

qtext = %d33 / ; Printable US-ASCII %d35-91 / ; characters not including %d93-126 / ; "\" or the quote character obs-qtext

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

obs-qtext = obs-NO-WS-CTL obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

obs-qp = "\" (%d0 / obs-NO-WS-CTL / LF / CR) obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

obs-qtext = obs-NO-WS-CTL obs-NO-WS-CTL = %d1-8 / ; US-ASCII control %d11 / ; characters that do not %d12 / ; include the carriage %d14-31 / ; return, line feed, and %d127 ; white space characters

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS] quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS]

obs-domain = atom *("." atom) atom = [CFWS] 1*atext [CFWS]

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

CFWS = (1*([FWS] comment) [FWS]) / FWS FWS = ([*WSP CRLF] 1*WSP) / obs-FWS ; Folding white space comment = "(" *([FWS] ccontent) [FWS] ")" ccontent = ctext / quoted-pair / comment ctext = %d33-39 / ; Printable US-ASCII %d42-91 / ; characters not including %d93-126 / ; "(", ")", or "\" obs-ctext

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

local-part = dot-atom / quoted-string / obs-local-part dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext) quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS] obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS]

IPv6-address-literal = "IPv6:" IPv6-addr IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present.

quoted-pair = ("\" (VCHAR / WSP)) / obs-qp VCHAR = %d33-126 ; visible (printing) characters WSP = SP / HTAB ; white space

domain = dot-atom / domain-literal / obs-domain dot-atom = [CFWS] dot-atom-text [CFWS] dot-atom-text = 1*atext *("." 1*atext)

obs-local-part = word *("." word) word = atom / quoted-string atom = [CFWS] 1*atext [CFWS] quoted-string = [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS]

obs-domain = atom *("." atom) atom = [CFWS] 1*atext [CFWS]

In the case of a top-level domain used by itself in an email address, a single string is used without any dots. This makes the requirement, described in more detail below, that only fully-qualified domain names appear in SMTP transactions on the public Internet, particularly important where top-level domains are involved.