Email resides outside of ISPConfig server

[cic]

We're attempting to use a sendmail script from the website to email a user who's email address is the same of the site (www.domain1.com - joe@domain1.com). However, when we attempt to send email to this user, we get the follownig message:
The Postfix program

<joe@domain1.com>: unknown user: "joe"
Reporting-MTA: dns; server1.example.com
X-Postfix-Queue-ID: 855A2218092
X-Postfix-Sender: rfc822; bob@domain2.com
Arrival-Date: Mon, 5 Nov 2007 17:03:55 -0500 (EST)

Final-Recipient: rfc822; joe@domain1.com
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; unknown user: "joe"

I know this is because Postfix thinks the domain is local to the server but how can I tell postfix that this particular domain's email server resides externally?

Thanks!
Troy

[falko]

Please select "External Mailserver" for domain1.com in ISPConfig (it's on the Options tab if domain1.com is the main domain of the web site; if it's a Co-Domain, you'll find the correct Options tab under Co-Domains).

[cic]

This was already set...

[falko]

Was it set for domain1.com or www.domain1.com? This is a difference...

[cic]

Ummmmmm..... From the admin login, I expand my reseller, expand the site and click on the site. The hostname is www and the domain is domain1.com. Under options, I select External MailServer for MailServer. Is this the correct location?

[falko]

Go to the Co-Domains tab, select domain1.com, and set it to "External Mailserver" as well (on the Options tab).