I've been using fetchmail/postfix with great success for a couple of
years now. I'm using Debian 2.2 (unstable - "sid"). Here's the overview:

1. My mail is hosted externally at a site where I've got various POP3
accounts set up, and several aliases pointing to each POP3 mailbox.

2. I use fetchmail to poll those POP3 accounts regularly, and send
messages to a few local accounts.

3. I use procmail locally with spamassassin for sorting and tagging
spam.

Lately, I've started getting errors from postfix:

Apr 25 06:55:19 mailhost fetchmail[19078]: SMTP error: 450
<clean-mailman-return
-188-mailings01=ttlexceeded.com@clean.cmsu.edu>: Sender address
rejected: Domain
 not found

("mailings01" is one of the aliases that points to a "mailings" user
account on the hosted server.)

A quick check confirms that "clean.cmsu.edu" does NOT resolve via DNS,
so the message makes sense. Despite the messages, the mail is being
delivered and sorted properly by procmail. Due to my setup, the mail has
already been received, so any spam is already in my hands. (OK, I want
to change this soon, but that's how it is for now.) I do use procmail
and spamassassin on the inside, and actual spam from bogus domains is
being tagged.

My configuration is unchanged, although I have updated (apt-get
dist-upgrade) the software recently. I'm wondering if any postfix gurus
can give me a hint on what features might have changed recently that
could cause these messages. They're not harmful, but alarming. I also
want to make sure I have my arms properly around Postfix before moving
it all in-house again.

Thanks,

- Bob