hawky--- via Postfix-users:
> Hi Wietse,
> 
> we are struggling with t-online.de: As you may know as SMTP client you 
> have to fulfill a bunch of requirements to be able to send mails to 
> t-online.de. From time to time we receive mails from t-online.de, but 
> can't answer.
> 
> So our idea is, in case the mail comes from t-online.de, we process and 
> delivery the mail but return an (meaningful) error message to the 
> t-online.de SMTP client.
> 
> This is meant as signal so that huge email provider can't dictate their 
> rules (besides SPF, DKIM, DMARC, etc. what we support) - and should 
> prevent a discussion like: "Just fulfill the requirements..." ;-)

If you want the business from t-online users, play by their rules
instead of giving you users a poor experience when they respond to
t-online email.

Otherwise, you could reject t-online email selectively:

smtpd_???_restrictions =
    ....
    reject_unauth_destination
    check_sender_access inline:{
        {t-online.de = reject_unverified_sender}}
    ...

If t-online.de blocks mail from your site, then the sender will
receive some gibberish that they can pass on to their support people.

        Wietse

> Thanks.
> -- 
> Hawky
> 
> 
> Am 20.09.2024 16:25 schrieb Wietse Venema via Postfix-users:
> > hawky--- via Postfix-users:
> >> Hi!
> >> 
> >> I'm looking for a way to process and deliver an incoming email, but
> >> return an error (with a meaningful) message to the client.
> >> 
> >> By looking at the SMTP status codes
> >> (https://en.wikipedia.org/wiki/List_of_SMTP_server_return_codes) I 
> >> don't
> >> see an obvious way to do that. But maybe it is possible in a 
> >> non-obvious
> >> way?
> >> 
> >> Is it possible?
> >> And if yes, can someone give me a hint howto implement?
> > 
> > What problem are you trying to solve? Deliver-and-reject ALL email,
> > or only SOME email?
> > 
> >     Wietse
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