qmail sees one domain's mail server as local when it is remote - DNS issue?

[thedust2010]

We have a problem where emails sent out internally using PHP's mail function are not getting to one of our domains. I'm guessing it's a DNS issue, because we have the DNS for this domain set up to use an external server. The thing is that email is working fine for this domain for all purposes except for email generated out of PHP. This is what our DNS looks like:

mail.mydomain.com.	A		xx.xx.xx.xxx
mydomain.com.		MX (10)		mail.mydomain.com.

xx.xx.xx.xxx is the IP of the external mail server. Does PHP require some extra MX records be set up, perhaps? When I check out the logs for qmail I see:

Apr 18 09:28:48 ns1 qmail: 1145370528.419949 delivery 1711: deferral: Sorry,_I_wasn't_able_to_establish_an_SMTP_connection._(#4.4.1)/

I've searched around the Net and tried some things but to no avail. Any ideas? I'm beating my head against a wall here...

 

[thedust2010]

NOTE: I've also confirmed that users on the same server cannot send to this domain as well. There must be something misconfigured with the DNS, but I'm not sure what...