David F. Skoll writes: > Well... any domain that sends mail must be prepared to receive it > also, if only to receive DSNs. > It is routine to block mail from a sending domain if it lacks MX, A and > AAAA records. Sendmail does that by default. > Blocking simply for a lack of MX records is wrong, however. > [Note: I refer to the domain part of the envelope sender here. It has > nothing to do with the domain name of the machine doing the sending.]
I agree on all accounts. If one receives a message but it later turns out that i cannot be delivered for some reason (e.g. quota exceeded, or mailbox unavailable for few days, ...), a DSN must be sent. If the domain specified in the envelope is indicating that it is not willing to receive any mail, than that message is effectively lost: neither recipient nor the sender can be notified. The only place that such loss can be prevented is to check that the sending domain has its MX or A or AAAA record, right away while the message is being received. It is prudent to reject such unbouncible mail right away, before even accepting it. This is the only opportunity that the sender can be made aware of the problem. Mark
