Christian Hertel said: > --
>>
> In our case, we have no possibility to verify either the sender address
> nor the recipient address (we only verify the domains), because it is
> just a mail relay server, not the server where the mailboxes are stored.
Tried recipient callouts? That works extremely well for us. The mailbox
servers spend most of their time 550'ing unknown recipients in response to
callouts, but that's much better than accepting them and generating
bounces.
The mailbox server needs to know how to reject unknown recipients at SMTP
time of course - no problem if it's Exim. We don't cater for downstream
servers that can't do this.
>
> So here's another example:
>
> * Mail comes in from kjfbhjkhs@??? (Domain can be resolved, no
> possibility to verify the local part)
>
> * Mail recipient is blablabla@??? (Domain is ok, we relay mail
> to it but again no possibility to verify the local part because the
> mailbox is stored on another server)
>
> * The destination mailserver rejects the mail in the smtp dialog with an
> '554 User unknown error', so generating the bounce message is our part.
>
> * Our bounce message could not be delivered to kjfbhjkhs@???
> because the yahoo mailserver rejects it in the smtp dialog again with an
> '554 User unknown' error. BUT - and that is the bad thing - our bounce
> message is not discarded but frozen by exim and held in our mailqueue.
> This happens thousand times so thousands of bounce mails were held in
> our mailqueue although they could never be delivered. Permanent means
> 'forget it, you could NEVER be deliver the mail'.
See above. Everything is rejected at SMTP time, zero undeliverable
bounces. It's the only sane way of handling things.
