Hello Cord,
On 17.09.23 12:02, Cord Beermann wrote:
tags 1050906 wontfix
severity 1050906 wishlist
thanks
Hallo! Du (Lee Garrett) hast geschrieben:
>From a user experience it's currently a bit cumbersome, as I'll send a mail,
wait 15 minutes to notice that it hasn't arrived, change a few things and
resend, wait another 15 minutes, etc. Which is quite an ineffective workflow.
Rejecting based on Mail content is discouraged by a RfC.
I was curious about that and re-read the current RFC related to SMTP. It clearly
states that the preference is ACCEPT > REJECT > BOUNCE > DISCARD. [0]
To quote:
"If they cannot be delivered, and cannot be rejected by the SMTP server
during the SMTP transaction, they should be "bounced" (returned with
non-delivery notification messages) as described above."
This also aligns with the best current practice as propagated from IRC #postfix
admins:
mantras:
1. do not accept mail that you do not intend to deliver.
2. do not drop mail.
3. do not use wildcards or catchalls.
4. do not forward mail to outside/third party systems
Accepting then discarding the mail would violate #1 and #2 of those mantras.
Discarding is only preferred over bouncing when the mail clearly contains
"hostile content" (spam, malware, etc.). I would not count a malformed signature
as such. In fact, discarding is strongly discouraged in RFC 5321:
"As discussed in Section 7.8 and Section 7.9 below, dropping mail
without notification of the sender is permitted in practice. However,
it is extremely dangerous and violates a long tradition and community
expectations that mail is either delivered or returned. If silent
message-dropping is misused, it could easily undermine confidence in
the reliability of the Internet's mail systems. So silent dropping of
messages should be considered only in those cases where there is very
high confidence that the messages are seriously fraudulent or otherwise
inappropriate."
I have also not found anything that could be read as rejecting mail based on
content is discouraged. That would also be very surprising as spam filtering and
rejection of such mail is widespread practice.
If we would reject misdirected mails to our lists we would produce
20000 backscatter mails daily to mostly innocent netizens.
As mentioned in the previous mail, rejecting during SMTP dialog will not result
in backscatter originating from Debian's MX. In contemporary setups, the MUA
will open a SMTP submission connection, the submission MX will keep the SMTP
dialog open and connect to the Debian MX, receive a reject, and backpropagate it
to the MUA. In practice the actual rejection message will be displayed in the
MUA, and the submission will fail.
If there is a temporary error (4xx), the submission MX might still queue the
mail, but in that case any DSN will originate from the submission MX, outside of
Debian's MX. And DSNs generated by other people's MX are IMHO not Debian's
problem domain.
So in the current state of Mail-Federation which is mostly driven by
spamming monopolists I don't see a working solution.
Yours,
Cord, Debian Listmaster of the day
[0] https://datatracker.ietf.org/doc/html/rfc5321#section-6.2
Best regards,
Lee
