When the receiver-SMTP accepts a piece of mail (by sending a "250 OK" message in response to DATA), it is accepting responsibility for delivering or relaying the message. It must take this responsibility seriously, i.e., it MUST NOT lose the message for frivolous reasons, e.g., because the host later crashes or because of a predictable resource shortage.
If there is a delivery failure after acceptance of a message, the receiver-SMTP MUST formulate and mail a notification message. This notification MUST be sent using a null ("<>") reverse path in the envelope; see Section 3.6 of RFC-821. The recipient of this notification SHOULD be the address from the envelope return path (or the Return-Path: line). However, if this address is null ("<>"), the receiver-SMTP MUST NOT send a notification. If the address is an explicit source route, it SHOULD be stripped down to its final hop.
For example, suppose that an error notification must be sent for a message that arrived with: "MAIL FROM:<@a,@b:user@d>". The notification message should be sent to: "RCPT TO:<user@d>".
Some delivery failures after the message is accepted by SMTP will be unavoidable. For example, it may be impossible for the receiver-SMTP to validate all the delivery addresses in RCPT command(s) due to a "soft" domain system error or because the target is a mailing list (see earlier discussion of RCPT).
To avoid receiving duplicate messages as the result of timeouts, a receiver-SMTP MUST seek to minimize the time required to respond to the final "." that ends a message transfer. See RFC-1047 [SMTP:4] for a discussion of this problem.