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...