Could somebody briefly explain me the difference between
"Auto-Submitted" and "Autosubmitted" MIME headers?
RFC 4021 (Registration of Mail and MIME Header Fields) talks only about
Autosubmitted, with reference to RFC 2156 ( MIXER (Mime Internet X.400
Enhanced Relay): Mapping between X.400 and RFC 822/MIME).
Autosubmitted: is a straight mapping of an X.400 message attribute into an RFC
822 header field. The semantics of that attribute are fairly specific to X.400
and have no well-defined meaning in Internet email. (I could go on and talk
about how well I think X.400 defines these things for its own use, but that's
not especially relevant here.)
On the other side, RFC 3834 (Recommendations for Automatic Responses to
Electronic Mail) "keywords for the Auto-Submitted header field", hence
Auto-Submitted shall have been defined before RFC 3834.
Nope. RFC 3834 defines Auto-submitted:, including providing ABNF. There is no
previous definition and no need for one.
RFC 5230 (Sieve Email Filtering: Vacation Extension) relies on
It uses it, yes. Relies on it is a bit of an overstatement, I think.
only Autosubmitted is listed.
Which is apropos of almost nothing, more's the pity. Although things have
improved in recent times, omissions in IANA registries are legion. Spurious
entries aren't exactly unknown either. IANA is always working to try and get
things up to date, but the problem is that too many things managed to slip
through the cracks for too long, making this a herculian task.