So. RFC 3501 (page 50-51), says that the localpart of a From: address is
matched case-insensitively when IMAP servers process SEARCH or UID
SEARCH commands. RFC 5321 says that SMTP servers process localparts
case-sensitively. Both rules go back essentially unchanged to very old
RFCs.
But that's the problem - this is not what RFC 5321 says. It says:
Consequently, and due to a
long history of problems when intermediate hosts have attempted to
optimize transport by modifying them, the local-part MUST be
interpreted and assigned semantics only by the host specified in the
domain part of the address.
SMTP server do stuff like expand lists all the time. For those tests to be
done competently some amount of interpretation of local parts may be needed,
such as ignoring the possibility that the local part is case sensitive.
A mailing list is a mediator (at the User level), not a relay at the SMTP
level.
THen what about autoforwarders with access controls?
It *is* the "host specified in the domain part of the address". So it should
interpret the local part.
Of the recipient address, sure. But not the originator address.
When I send this message to ietf-smtp(_at_)imc(_dot_)org, only imc.org knows
how to
interpret the local part. RFC 5321 is not in conflict with reality, at least
in this example.
When you send mail to ietf-smtp(_at_)imc(_dot_)org, a check is made to see if
you're a
subscriber to the list. I doubt that your address is in imc.org and that check
is probably done in a case-insensitive way.
Replace ietf-smtp(_at_)imc(_dot_)org with an autoforwarder with access control,
and now
RFC 5321 *is* in conflict with reality.
Ned