free smtp to coincide with xpsp2 and apache2

Hi,

I was originally using postcast server to send emails from our intranet site out to my email. It worked (most of the time) for the last few months, now it won't work at all. The other weird thing is, If I set up IIS on another computer, and try to send emails through it (via php sendmail) they never make it, but sendmail doesn't die so it has to be something with my network config. I'd rather not disable the firewall on this machine. So my question is, does anyone know of a good smtp program that's compatible with smtp (and no issues with apache like IIS does). Thanks, i've been scouring for a program like this.