I'd like to have a method to allow Exim to return a 4xx temporary
failure to a mail only _once_. If a defer_once result is obtained for a
message which has already been seen (using Message-ID as a key and maybe
md5sum of body if there's no Message-ID), the message should be
accepted.
I'd use this in conjunction with content filtering. Those mails which
are only _slightly_ suspicious can be deferred once and if the sender
makes a second attempt to deliver them, we're then slightly more
inclined to believe it's a genuine mail.
I know it can be hacked up with SpamAssassin and sa-exim, but I think
it'd be a lot cleaner and more generally useful if Exim could do it for
itself.
