I'm aware of this. Unluckily, some of our customers tend to make many
mistakes so the mail bounce (and they quite often misconfigure their source
address so the mail double-bounces).
Therefore I would like to push the checks at smtp time.
Currently courier does:
- validate sender domain
- validate recipient address, if it's local.
What would I like to add:
- validate recipient domain
- validate sender address
And I'd like to change address validation that it would be validated, if
it's known, not only if it's local.
This could give us advantage on MX relays, which could e.g. mirror
recipients database using ldap...
All of this incurs non-trivial per message overhead. Issuing a DNS query,
and/or querying the local account database, is not free, and the penalty
now applies to every message. A judgement call needs to be made whether the
additional overhead is justified by the value added.
As the old saying goes: build an idiot-proof process, and nature will evolve
a better idiot.