I'd add to Brent's answer that there's an option in Sendmail 8.6.9 (and maybe in other versions, I don't know) to send a copy of *all* errors to someone (eg to Postmaster) -
# who (if anyone) should get extra copies of error messages
Maybe this is contributing to the problem?
From: Brent@GreatCircle.COM (Brent Chapman)
Date: Mon, 6 Feb 1995 22:55:16 -0500
Subject: Re: Checking validity of e-mail addresses
>I then get an unknown host message delivered to both
>the list-owner and the postmaster. Does anyone know how to avoid the mail
>being returned to the postmaster? Thanks.
That's easy: set up an alias like so:
If there is an "owner-*" alias for an alias that's having problems,
Sendmail will send bounces there instead of to Postmaster. The comma at
the end is to work around a bug (or "feature", depending on who you ask) in