Maximum internet email Message-ID length
If there's one thing I've learned about email, it must be that it's a massively distributed system for fuzzing email software. That is, no matter what the RFCs say, you will find emails violating them, some email software coping and some failing. I think most will limp along with the robustness principle in mind.
With that out of the way, I think the maximum RFC compliant Message-ID length is 995 characters.
The maximum line length per the RFC you cite is 998 characters. That would include the "Message-ID:" field name, but you can do line folding between the field name and the field body. The line containing the actual Message-ID would then contain a space (the folding whitespace), "<", Message-ID, and ">". Semantically, the angle brackets are not part of the Message-ID. Therefore you end up with a maximum of 998 - 3 = 995 characters.
Actually there's no limit
RFC2822 defines these productions:
message-id = "Message-ID:" msg-id CRLF
msg-id = [CFWS] "<" id-left "@" id-right ">" [CFWS]
id-left = dot-atom-text / no-fold-quote / obs-id-left
obs-id-left = local-part
local-part = dot-atom / quoted-string / obs-local-part
quoted-string = [CFWS]
DQUOTE *([FWS] qcontent) [FWS] DQUOTE
[CFWS]
CFWS = *([FWS] comment) (([FWS] comment) / FWS)
FWS = ([*WSP CRLF] 1*WSP) / ; Folding white space
So id-left
can be local-part
which can be quoted-string
(and thus have multiple FWS
)
so you can fold it as many times as needed to fit any arbitrary
length of payload and still comply with the restrictions given
by the RFC.