Sendmail - route mail off server for hosted domain
I'm running sendmail on a small CentOS server, where there is only one hosted domain. This domain's http content is served from this server, but mail is served from another server, to which the MX record for this domain points (DNS is hosted on a third server).
This server is able to send mail successfully to any addresses not on this domain, but can't send mail to addresses on the domain.
Other servers are able to send mail to this domain.
Thus, it would seem the system is trying to deliver the mail locally, where I need it to deliver them to another server. How can I force all mail to be routed off the server?
Mail is sent by php_mail()