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.