I'm really confused when it comes to the format of Content-Id headers in message parts.
It seems to me that only RFC 2045 covers the format of the header, however briefly:
In constructing a high-level user agent, it may be desirable to allow one body to make reference to another. Accordingly, bodies may be
labelled using the "Content-ID" header field, which is syntactically
identical to the "Message-ID" header field:id := "Content-ID" ":" msg-idLike the Message-ID values, Content-ID values must be generated to be world-unique.
RFC 2822 explains the format of a msg-id token like so:
The message identifier (msg-id) is similar in syntax to an angle-addr construct without the internal CFWS.
message-id = "Message-ID:" msg-id CRLF
in-reply-to = "In-Reply-To:" 1*msg-id CRLF
references = "References:" 1*msg-id CRLF
msg-id = [CFWS] "<" id-left "@" id-right ">" [CFWS]
id-left = dot-atom-text / no-fold-quote / obs-id-left
id-right = dot-atom-text / no-fold-literal / obs-id-right
no-fold-quote = DQUOTE *(qtext / quoted-pair) DQUOTE
no-fold-literal = "[" *(dtext / quoted-pair) "]"
Long story short: it includes the at ('@') symbol, just like the Message-Id header of a message. However, almost all reader-friendly articles on MIME format give examples of Content-Id without the at symbol (including not-really-global identifiers like myimagecid or inlineimage001 as well as randomly generated UUIDS without the at symbol). They would surely stress the importance of the '@' symbol if that would be necessary, just like they do with the Message-Id header, right? Right?
I've run some tests on real-world email clients and see how they compose emails with embedded inline images:
- Thunderbird generates identifiers with the at symbol. Example:
[email protected] - Gmail generates identifiers without such symbol and with no domain part. Example:
ii_abc1234x0_12345ab12abcdefa
I didn't test any more email clients (if someone did, it'd be great to complete the list above), but these two already show the striking difference. Google not obeying RFC standards? It sure looks smelly and I want to know whether that's because I missed something, or because the format isn't really that important after all (which in the long run feels rather disturbing). I'm also interested in checking how many popular email clients actually discard the 'at' symbol.
