Php Sendmail does not send to external email addresses

[Austin]

I have Zimbra installed on a separate server (Godaddy dedicated server) and a MX record setup on a webserver (also a godaddy dedicated server) pointing to the Zimbra server. Everything sends out and receives email on the Zimbra server. On the webserver, it does not send out mail through php sendmail. How do I route all emails from the webserver through the Zimbra server?

[Austin]

I have found the problem with the sendmail. It was in the php coding of the mail script not sending a special parameter in the header.

If I send email to anyname@mydomain on my webserver it just looks for the email address on the webserver and does not forward it to my zimbra server. Is there something I have to disable?