I think to make active a mail server on your os, sendmail is the most able to realize this task. It's hard to configure but default configuration under MandrakeLinux and RedHat are good for that. Last month, I had configured a network with WinXP, Win2000, WinNT 4 WK and RedHat 8.0 as server proxy and mail to filter vbs, exe ... attached file on e-mail received by Win. I had used squid, fetchmail, imap3, and SENDMAIL but I needn't to reconfigure it.
Sendmail is very good software. the only good alternative for sendmail (it's what i think) is PostFix, very simple to configure and you can find him in All RedHat distributions CDs.
*Sorry for word mistakes : I'm French