PHP Forum has trouble sending emails

I installed vBulletin on my Mountain Biking web site. It went VERY smooth. Very little configuration to get it to work. I send my self an email using the mail test and it all worked. So I let it rip. Turns out, people that are trying to join, cannot. My mail server, mysql, and apache are all the same machine. Mails sent to myself go through fine. Mails sent to any other email address never arrive anywhere. Mails I send through my mail client go through fine though. I can't figure it out. And I can't find ANY mail server setting anywhere. The only thing I can come up with is that it is useing sendmail and sendmail isn't configured right. The mail server running on the machine that I use is AppleMail Server that comes with Mac OS X Server. It is a SMTP server. Back when I was on UBB, I couldn't get it to send emails through sendmail, finally I tried switching it to SMTP server, gave the machine its own IP, and it started sending out emails no problem. Are there any such settings for PHP/MySQL/vB ? I'd rather using my normal SMTP server than configure Sendmail. But I don't really know what my problem is or what my options are. Please help. Thanks.

Check this out. It may not be what you are looking for but I had the same problem and got sendmail working.
If you have Verizon you'll probably need something like this.

http://ww2.4linuxworld.com:8008/documents/Sendmail_as_client.php

My web site is new so leave a note on the forum and Ill try to help

I don't want or need to use my ISP's mail servers, I have my own running on the same machine. I want to use that.