Postfix Loops Back error and weird email headers

[reimagine]

I'm getting the "loops back" error on my new ISPConfig setup (running on CentOS). Sometimes emails go through properly, other times I receive the following error message. In this example, email was sent to ben@domain1.com.

This is the Postfix program at host domain2.com.

I'm sorry to have to inform you that your message could not be
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The Postfix program

<web26_ben@domain3.com> (expanded from < ben@domain1.com>): mail for
domain3.com loops back to myself

The headers of emails that make it through also display multiple domains. For example:

X-Original-To: ben@domain1.com
Delivered-To: web26_ben@domain2.com
Received: from ug-out-1314.google.com (ug-out-1314.google.com [66.249.92.168]) by www.domain3.com (Postfix) with ESMTP id B7AEBF6045A for <ben@domain1.com>; Wed, 26 Jul 2006 10:49:41 -0400 (EDT)

Any ideas of where to look as I try to fix this? It seems that the main.cf file for Postfix is set up properly:

virtual_maps = hash:/etc/postfix/virtusertable

mydestination = /etc/postfix/local-host-names

[till]

Please add:

domain3.com

(the domain that is mentioned in the error message)

at the end of the file:

/etc/postfix/local-host-names

[reimagine]

Thanks for the info. I looked at the site settings. domain3.com was a co-domain on another site. I had a www version setup (so www.domain3.com was listed in local-host-names), but no non-www version (so domain3.com was not listed there). I went ahead and set up domain3.com (no www) as a co-domain and it's now showing up it the local-host-names file. I did a postfix reload and I'll keep an eye out for any more errors.

How should I set up co-domains if they aren't going to be used for email. My only options are local mail server or external mail server ... there is no "none" option.

Thanks!

[reimagine]

Here is another example of a "loops back" error that I'm getting, but with a slight variation:

Jul 27 20:54:53 ns1 postfix/smtp[19638]: E3C74F604AA: to=<web26_ben@mail.domain1.com>, orig_to=<ben@domain1.com>, relay=none, delay=7053, status=SOFTBOUNCE (mail for mail.domain1.com loops back to myself)

"domain1" is the same name in both instances, but somehow "mail" gets added to the beginning of the domain name. There are no mail.domains listed in the local-host-names file.

The DNS settings for this domain are as follows:

A Record
ip.ad.dr.ess www
ip.ad.dr.ess mail

Mailserver
mail.domain1.com @

Thanks.

[till]

Please add mail.domain1.com to the local-host-names file too.