Quoting RFC 2821 section 6.1:
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. Obviously, nothing in this section can or should
prohibit local decisions (i.e., as part of the same system
environment as the receiver-SMTP) to log or otherwise transmit
information about null address events locally if that is desired.
If
the address is an explicit source route, it MUST be stripped down
to
its final hop.
XMail doesn't set the "Return-Path" header at all, so the non-
delivery reports are not following the RFC.
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