The fundamental problem is that SMTP mandates accepting multiple recipients per transaction, but only a single status code when completing the transaction. Tran

The fundamental problem is that SMTP mandates accepting multiple recipients per transaction, but only a single status code when completing the transaction. Transaction level rejection is incompatible with per-user rules that filter the message body. Ideally what's needed is a mandatory, pervasively supported SMTP extension that permits per-recipient status codes, such as LMTP provides. More feasible is permitting MTAs to accept only a single recipient per transaction, but even that is difficult given the huge installed base of MTAs and libraries.