OS X Server requires DNS services on your LAN. That means local DNS services, and not references out to ISP DNS. OS X Server gets squirrelly if it doesn't have local and valid DNS, as do many other recent servers, as DNS is a fundamental part of network authentication and security.
To check DNS, launch Applications > Utilities > Terminal.app and issue the following non-disruptive diagnostic command:
sudo changeip -checkhostname
Enter your administrative password when prompted. That command will show information about the host, and then an indication that there are no changes required, or that there are issues with the DNS or host name set-up.
Here are details on how to set up DNS services on OS X Server. With 10.8, you'll likely need to use Server.app and the "advanced" tab stuff in the DNS set-up, but the rest is similar.
I recommend spending the US$10 per year and getting a real and registered domain name for your LAN.
Once DNS is sorted out, your mail server will need to know its internal network name and its external name, as if the host doesn't know its own names, it'll think the message is a relay and (if properly configured) reject the message. You don't want to become an open relay, as those open relays will be found and your network link and your server will become filled with folks shipping spam through your server.
If all you're doing is sending mail from php, then you can shut down the mail server and install and configure something like phpmailer tool.
FWIW(1): You've expurgated details, so I can't check your public DNS set-up.
FWIW(2): The server.com domain is a real and registered domain, and probably not one you have registered. If you're really using that, please don't. If you've used that to substitute for the domain you're using, please consider using the example.com, example.org or example.net domain names; the domains that are reserved for this usage, and reserved for use in documentation.
