Email resides outside of ISPConfig server

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 - [email protected]). However, when we attempt to send email to this user, we get the follownig message:
The Postfix program

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

Final-Recipient: rfc822; [email protected]
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

 

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).

 

This was already set...

 

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

 

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?

 

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