PHP mail ... Not working to own domain

I would guess there may be a setting on the mail server that is causing this. Are the email addresses for your domain registered in some way? Yahoo for instance checks yahoo email addresses against their registration to see if it's a real email address. That's the only thing I can think of. I've done the same as you're doing and haven't had any issues.

 

Assuming your MTA is sendmail and there are no issues with firewall or anti-spam filters (and the address you're testing is a valid mail account) then ask your system admin to ensure the following (intended for Linux):

[FONT=Arial, Helvetica, sans-serif]Edit the /etc/mail/access file to include IP addresses or domains for which you will relay mail. By default, no other hosts other than localhost can send mail to outside destinations. If you want to provide outbound smtp functionality to other computers on your network, include the network addresses for these other computers here. If you want to allow relaying from hosts outside of your network, enter their IPs or domains here.

For example:

localhost.localdomain RELAY
localhost RELAY
127.0.0.1 RELAY

If you only want to relay mail for users who authenticate with a username and password, use SMTP Authentication.
To enable your changes, run the following as root: # makemap hash /etc/mail/access.db < /etc/mail/access[/FONT]