Forum Moderators: phranque
I use Merak as my main SMTP server, but it's not free... I'm currently using hMail as my backup MX on a separate box. There are others, but this'll get you started on the search :)
XP Pro has a built-in SMTP server, but I wouldn't even bother with it.
Give us the exact error you're getting, and one of us will likely be able to help more.