David MacQuigg wrote:
Is it permissible to abort data transfer before the end of data, say
after you have received all headers, and you know the DKIM sig is bad?
RFC-5321 section 3.3 says:
If the verb is initially accepted and the 354 reply issued, the DATA
command should fail only if the mail transaction was incomplete (for
example, no recipients), if resources were unavailable (including, of
course, the server unexpectedly becoming unavailable), or if the
server determines that the message should be rejected for policy or
Failed DKIM would be a policy reason.
Independent of DKIM considerations, in our experience where we had a
condition to check the accumulated block transfer bytes received
exceeding local policy limited and dropping the line created
It was better to wait until the DATA was completed and issue a 55x
response in order to stop the retransmissions.
In regard to DKIM policy based rejections, I recommend:
- Record this failed state detected at DATA
- Accept the message,
- Then under RFC 5617 silently discard the message.
The reason is because if the sender is a mailing list, a SMTP level
rejects could initiate the the mailing list server to being sending
"Last Warning" Subscription Removal notifications after a number of
attempts are made.
In order words, any intermediary (re)signer who is not supporting RFC
5617 intentionally or otherwise and continues to forwarded a broken
ADSP protected domain message is subject to down link rejections at
receivers who support DKIM/ADSP.
To mitigate this, the receiver should accept the message and discard
it instead of creating a potential of harming the membership list.
For our Mailing list Server which will honor RFC 5617, because of this
potential conflict, we will filter all submissions from ADSP domains.