[Dovecot] Some Dovecot LDA version sends MDN rather than DSN for overquota.

[Mark Sapiro]

It appears that there is some version of the Dovecot LDA that is
sending an RFC 3798 MDN rather than an RFC 3464 DSN to report
non-delivery to a user who is over quota.

See <https://bugs.launchpad.net/mailman/+bug/693134> for a discussion
of this and
<https://bugs.launchpad.net/mailman/+bug/693134/+attachment/1773018/+files/Automatically%20rejected%20mail.msg>
for the actual MDN message.

The issue is that apparently a user was over quota and Dovecot returned
an MDN with a disposition of deleted. This is intended to mean the the
mail was delivered to the user but the user deleted it without reading
it. If the message was actually not delivered, this should be reported
to the MTA so it can return an appropriate SMTP status, or the LDA
should send an RFC 3464 DSN with an action of failed.

Note that I do not actually use the Dovecot LDA nor do I have any
access to the system where the notice in the bug report was received,
so I may have incomplete information and this may not be relevant to
current versions of Dovecot, but I think it is wrong for Dovecot to
report over quota non-delivery via an MDN.

-- 
Mark Sapiro <mark at msapiro.net>        The highway is for gamblers,
San Francisco Bay Area, California    better use your sense - B. Dylan