Update to Internet Message Format to Allow Group Syntax in the "From:" and "Sender:" Header Fields
The Internet Message Format (RFC 5322) allows "group" syntax in some email header fields, such as "To:" and "CC:", but not in "From:" or "Sender:". This document updates RFC 5322 to relax that restriction, allowing group syntax in those latter fields, as well as in "Resent-From:" and "Resent-Sender:", in certain situations.
What it is
Amends RFC 5322 to allow group syntax in the From: and Sender: header fields.
What it solves
Group syntax, such as an empty group used to hide recipients, was legal in To: and Cc: but not in From:, and real messages used it anyway.
What it means to operate
A small amendment with a real consequence for parsers: a From: field may legally contain a group construct, so a parser assuming a single mailbox can fail on valid mail. It is also an example of why the amendment list on an RFC matters: RFC 5322 is current and this changed part of it.
Header analyser parses a real message and says what each header is claiming. Open header analyser
Normative requirements
Every sentence in this RFC carrying an RFC 2119 keyword, with the section it came from.
2.1 Replacement of RFC 5322, Section 3.6.2. Originator Fields
- MUSTIf the from field contains more than one mailbox specification (including all mailboxes included in any groups), then the sender field, containing the field name "Sender" and a single address, MUST appear in the message.
- SHOULD NOTThe from field and the sender field SHOULD NOT use group syntax; rather, the from field SHOULD use only the mailbox-list syntax and the sender field SHOULD use only mailbox syntax (see RFC 6854, Section 3).
- MUST NOTIf the sender field uses group syntax, the group MUST NOT contain more than one mailbox.
- MAYIn either case, an optional reply-to field MAY also be included, which contains the field name "Reply-To" and a comma-separated list of one or more addresses.
- SHOULD NOTIf the originator of the message can be indicated by a single mailbox and the author and transmitter are identical, the "Sender:" field SHOULD NOT be used.
- SHOULDOtherwise, both fields SHOULD appear.
- SHOULDIn the absence of the "Reply-To:" field, replies SHOULD by default be sent to the mailbox(es) specified in the "From:" field unless otherwise specified by the person composing the reply.
- SHOULD NOTIn all cases, the "From:" field SHOULD NOT contain any mailbox that does not belong to the author(s) of the message.
3 Applicability Statement
- SHOULD NOTIn particular, user agents SHOULD NOT permit the use of groups in those fields in outgoing messages.