4. Obsolete Syntax
Earlier versions of this specification allowed for different (usually more liberal) syntax than is allowed in this version. Also, there have been syntactic elements used in messages on the Internet whose interpretations have never been documented. Though these syntactic forms MUST NOT be generated according to the grammar in section 3, they MUST be accepted and parsed by a conformant receiver. This section documents many of these syntactic elements. Taking the grammar in section 3 and adding the definitions presented in this section will result in the grammar to use for the interpretation of messages.
Note: This section identifies syntactic forms that any implementation MUST reasonably interpret. However, there are certainly Internet messages that do not conform to even the additional syntax given in this section. The fact that a particular form does not appear in any section of this document is not justification for computer programs to crash or for malformed data to be irretrievably lost by any implementation. It is up to the implementation to deal with messages robustly.
One important difference between the obsolete (interpreting) and the current (generating) syntax is that in structured header field bodies (i.e., between the colon and the CRLF of any structured header field), white space characters, including folding white space, and comments could be freely inserted between any syntactic tokens. This allowed many complex forms that have proven difficult for some implementations to parse.
Another key difference between the obsolete and the current syntax is that the rule in section 3.2.2 regarding lines composed entirely of white space in comments and folding white space does not apply. See the discussion of folding white space in section 4.2 below.
Finally, certain characters that were formerly allowed in messages appear in this section. The NUL character (ASCII value 0) was once allowed, but is no longer for compatibility reasons. Similarly, US- ASCII control characters other than CR, LF, SP, and HTAB (ASCII values 1 through 8, 11, 12, 14 through 31, and 127) were allowed to appear in header field bodies. CR and LF were allowed to appear in messages other than as CRLF; this use is also shown here.
Other differences in syntax and semantics are noted in the following sections.
4.1. Miscellaneous Obsolete Tokens
These syntactic elements are used elsewhere in the obsolete syntax or in the main syntax. Bare CR, bare LF, and NUL are added to obs-qp, obs-body, and obs-unstruct. US-ASCII control characters are added to obs-qp, obs-unstruct, obs-ctext, and obs-qtext. The period character is added to obs-phrase. The obs-phrase-list provides for a (potentially empty) comma-separated list of phrases that may include "null" elements. That is, there could be two or more commas in such a list with nothing in between them, or commas at the beginning or end of the list.
Note: The "period" (or "full stop") character (".") in obs-phrase is not a form that was allowed in earlier versions of this or any other specification. Period (nor any other character from specials) was not allowed in phrase because it introduced a parsing difficulty distinguishing between phrases and portions of an addr-spec (see section 4.4). It appears here because the period character is currently used in many messages in the display-name portion of addresses, especially for initials in names, and therefore must be interpreted properly.
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-ctext = obs-NO-WS-CTL
obs-qtext = obs-NO-WS-CTL
obs-utext = %d0 / obs-NO-WS-CTL / VCHAR
obs-qp = "\" (%d0 / obs-NO-WS-CTL / LF / CR)
obs-body = *((*LF *CR *((%d0 / text) *LF *CR)) / CRLF)
obs-unstruct = *((*LF *CR *(obs-utext *LF *CR)) / FWS)
obs-phrase = word *(word / "." / CFWS)
obs-phrase-list = [phrase / CFWS] *("," [phrase / CFWS])
Bare CR and bare LF appear in messages with two different meanings. In many cases, bare CR or bare LF are used improperly instead of CRLF to indicate line separators. In other cases, bare CR and bare LF are used simply as US-ASCII control characters with their traditional ASCII meanings.
4.2. Obsolete Folding White Space
In the obsolete syntax, any amount of folding white space MAY be inserted where the obs-FWS rule is allowed. This creates the possibility of having two consecutive "folds" in a line, and therefore the possibility that a line which makes up a folded header field could be composed entirely of white space.
obs-FWS = 1*WSP *(CRLF 1*WSP)
4.3. Obsolete Date and Time
The syntax for the obsolete date format allows a 2 digit year in the date field and allows for a list of alphabetic time zone specifiers that were used in earlier versions of this specification. It also permits comments and folding white space between many of the tokens.
obs-day-of-week = [CFWS] day-name [CFWS]
obs-day = [CFWS] 1*2DIGIT [CFWS]
obs-year = [CFWS] 2*DIGIT [CFWS]
obs-hour = [CFWS] 2DIGIT [CFWS]
obs-minute = [CFWS] 2DIGIT [CFWS]
obs-second = [CFWS] 2DIGIT [CFWS]
obs-zone = "UT" / "GMT" / ; Universal Time
; North American UT
; offsets
"EST" / "EDT" / ; Eastern: - 5/ - 4
"CST" / "CDT" / ; Central: - 6/ - 5
"MST" / "MDT" / ; Mountain: - 7/ - 6
"PST" / "PDT" / ; Pacific: - 8/ - 7
;
%d65-73 / ; Military zones - "A"
%d75-90 / ; through "I" and "K"
%d97-105 / ; through "Z", both
%d107-122 ; upper and lower case
Where a two or three digit year occurs in a date, the year is to be interpreted as follows: If a two digit year is encountered whose value is between 00 and 49, the year is interpreted by adding 2000, ending up with a value between 2000 and 2049. If a two digit year is encountered with a value between 50 and 99, or any three digit year is encountered, the year is interpreted by adding 1900.
In the obsolete time zone, "UT" and "GMT" are indications of "Universal Time" and "Greenwich Mean Time", respectively, and are both semantically identical to "+0000".
The remaining three character zones are the US time zones. The first letter, "E", "C", "M", or "P" stands for "Eastern", "Central", "Mountain", and "Pacific". The second letter is either "S" for "Standard" time, or "D" for "Daylight Savings" (or summer) time. Their interpretations are as follows:
EDT is semantically equivalent to -0400 EST is semantically equivalent to -0500 CDT is semantically equivalent to -0500 CST is semantically equivalent to -0600 MDT is semantically equivalent to -0600 MST is semantically equivalent to -0700 PDT is semantically equivalent to -0700 PST is semantically equivalent to -0800
The 1 character military time zones were defined in a non-standard way in [RFC0822] and are therefore unpredictable in their meaning. The original definitions of the military zones "A" through "I" are equivalent to "+0100" through "+0900", respectively; "K", "L", and "M" are equivalent to "+1000", "+1100", and "+1200", respectively; "N" through "Y" are equivalent to "-0100" through "-1200". respectively; and "Z" is equivalent to "+0000". However, because of the error in [RFC0822], they SHOULD all be considered equivalent to "-0000" unless there is out-of-band information confirming their meaning.
Other multi-character (usually between 3 and 5) alphabetic time zones have been used in Internet messages. Any such time zone whose meaning is not known SHOULD be considered equivalent to "-0000" unless there is out-of-band information confirming their meaning.
4.4. Obsolete Addressing
There are four primary differences in addressing. First, mailbox addresses were allowed to have a route portion before the addr-spec when enclosed in "<" and ">". The route is simply a comma-separated list of domain names, each preceded by "@", and the list terminated by a colon. Second, CFWS were allowed between the period-separated elements of local-part and domain (i.e., dot-atom was not used). In addition, local-part is allowed to contain quoted-string in addition to just atom. Third, mailbox-list and address-list were allowed to have "null" members. That is, there could be two or more commas in such a list with nothing in between them, or commas at the beginning or end of the list. Finally, US-ASCII control characters and quoted- pairs were allowed in domain literals and are added here.
obs-angle-addr = [CFWS] "<" obs-route addr-spec ">" [CFWS]
obs-route = obs-domain-list ":"
obs-domain-list = *(CFWS / ",") "@" domain
*("," [CFWS] ["@" domain])
obs-mbox-list = *([CFWS] ",") mailbox *("," [mailbox / CFWS])
obs-addr-list = *([CFWS] ",") address *("," [address / CFWS])
obs-group-list = 1*([CFWS] ",") [CFWS]
obs-local-part = word *("." word)
obs-domain = atom *("." atom)
obs-dtext = obs-NO-WS-CTL / quoted-pair
When interpreting addresses, the route portion SHOULD be ignored.
4.5. Obsolete Header Fields
Syntactically, the primary difference in the obsolete field syntax is that it allows multiple occurrences of any of the fields and they may occur in any order. Also, any amount of white space is allowed before the ":" at the end of the field name.
obs-fields = *(obs-return /
obs-received /
obs-orig-date /
obs-from /
obs-sender /
obs-reply-to /
obs-to /
obs-cc /
obs-bcc /
obs-message-id /
obs-in-reply-to /
obs-references /
obs-subject /
obs-comments /
obs-keywords /
obs-resent-date /
obs-resent-from /
obs-resent-send /
obs-resent-rply /
obs-resent-to /
obs-resent-cc /
obs-resent-bcc /
obs-resent-mid /
obs-optional)
Except for destination address fields (described in section 4.5.3), the interpretation of multiple occurrences of fields is unspecified. Also, the interpretation of trace fields and resent fields that do not occur in blocks prepended to the message is unspecified as well. Unless otherwise noted in the following sections, interpretation of other fields is identical to the interpretation of their non-obsolete counterparts in section 3.
4.5.1. Obsolete Origination Date Field
obs-orig-date = "Date" *WSP ":" date-time CRLF
4.5.2. Obsolete Originator Fields
obs-from = "From" *WSP ":" mailbox-list CRLF
obs-sender = "Sender" *WSP ":" mailbox CRLF
obs-reply-to = "Reply-To" *WSP ":" address-list CRLF
4.5.3. Obsolete Destination Address Fields
obs-to = "To" *WSP ":" address-list CRLF
obs-cc = "Cc" *WSP ":" address-list CRLF
obs-bcc = "Bcc" *WSP ":"
(address-list / (*([CFWS] ",") [CFWS])) CRLF
When multiple occurrences of destination address fields occur in a message, they SHOULD be treated as if the address list in the first occurrence of the field is combined with the address lists of the subsequent occurrences by adding a comma and concatenating.
4.5.4. Obsolete Identification Fields
The obsolete "In-Reply-To:" and "References:" fields differ from the current syntax in that they allow phrase (words or quoted strings) to appear. The obsolete forms of the left and right sides of msg-id allow interspersed CFWS, making them syntactically identical to local-part and domain, respectively.
obs-message-id = "Message-ID" *WSP ":" msg-id CRLF
obs-in-reply-to = "In-Reply-To" *WSP ":" *(phrase / msg-id) CRLF
obs-references = "References" *WSP ":" *(phrase / msg-id) CRLF
obs-id-left = local-part
obs-id-right = domain
For purposes of interpretation, the phrases in the "In-Reply-To:" and "References:" fields are ignored.
Semantically, none of the optional CFWS in the local-part and the domain is part of the obs-id-left and obs-id-right, respectively.
4.5.5. Obsolete Informational Fields
obs-subject = "Subject" *WSP ":" unstructured CRLF
obs-comments = "Comments" *WSP ":" unstructured CRLF
obs-keywords = "Keywords" *WSP ":" obs-phrase-list CRLF
4.5.6. Obsolete Resent Fields
The obsolete syntax adds a "Resent-Reply-To:" field, which consists of the field name, the optional comments and folding white space, the colon, and a comma separated list of addresses.
obs-resent-from = "Resent-From" *WSP ":" mailbox-list CRLF
obs-resent-send = "Resent-Sender" *WSP ":" mailbox CRLF
obs-resent-date = "Resent-Date" *WSP ":" date-time CRLF
obs-resent-to = "Resent-To" *WSP ":" address-list CRLF
obs-resent-cc = "Resent-Cc" *WSP ":" address-list CRLF
obs-resent-bcc = "Resent-Bcc" *WSP ":"
(address-list / (*([CFWS] ",") [CFWS])) CRLF
obs-resent-mid = "Resent-Message-ID" *WSP ":" msg-id CRLF
obs-resent-rply = "Resent-Reply-To" *WSP ":" address-list CRLF
As with other resent fields, the "Resent-Reply-To:" field is to be treated as trace information only.
4.5.7. Obsolete Trace Fields
The obs-return and obs-received are again given here as template definitions, just as return and received are in section 3. Their full syntax is given in [RFC5321].
obs-return = "Return-Path" *WSP ":" path CRLF
obs-received = "Received" *WSP ":" *received-token CRLF
4.5.8. Obsolete optional fields
obs-optional = field-name *WSP ":" unstructured CRLF