SMTP Watcher   
http://www.sb-software.com

SMTP Watcher is a tool designed to monitor your SMTP servers. This program is mainly intended for use by web administrators and server administrators. Your SMTP server is the one that is responsible for incoming mail. If it goes down, then people won't be able to send you mail. SMTP Watcher sits in the windows system tray and continuously polls your SMTP server to make sure it is online.

Protoport Sendmail Server   
http://www.vtapi.com

Powerful direct remailer software actr as SMTP relay. It puts email messages directly to mailbox of recipient. ISP's SMTP server is not involved. The program will query DNS for MX records, and deliver emails to so-called mail exhange servers. This is much more faster and more reliable. You can even use HTTP proxy server for sending emails. This is useful if you are behind corporate firewall, for example.

SoftCab Sendmail Server   
http://www.vtapi.com

Powerful direct remailer software actr as SMTP relay. It puts email messages directly to mailbox of recipient. ISP's SMTP server is not involved. The program will query DNS for MX records, and deliver emails to so-called mail exhange servers. This is much more faster and more reliable. You can even use HTTP proxy server for sending emails. This is useful if you are behind corporate firewall, for example.