Trigve Siver:
> Thanks for reply.
>
> Could some post the except from standard where this is defined to be able to
> argue with relayhost provider, please?
RFC 5321 is the current SMTP definition.
6.1. Reliable Delivery and Replies by Email
...
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. The recipient of this notification MUST 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.
