I run a social network on my web server, with about 15,000 members right now. My administration section let's me Mass Email all my users. Currently it uses the built in PHP mail function.

What is the best way to congfigure my server to bypass spam? Can I install anything on the server? Or should I just make the social network use SMTP? The admin panel lets me choose SMTP or built-in mail function.

I'm not to familiar with mailing from servers, as I usually use Aweber for my mailing, but I cannot use Aweber for this as they will not let me just import 15,000 emails.

Let me know, thanks.