The qmail program uses
a Delivered-To: header to trace all the alias and
mailing list expansions through which an email message passes. This
is similar to the way Received: headers are used
to trace machine hops. When qmail expands a
mailing list, it adds a Delivered-To: header to
the top of the message:
Delivered-To: list@host
If an identical header is already present, qmail
bounces the message. This prevents several kinds of mail loops. (Note
that the SmartList program supports an
X-Loop: header with the same function.)
The Delivered-To: header should never be declared
in the configuration file.
