What are some of the best opensource SMTP servers ?

Technology Tidbits

I really like qmail. Compiling it is somewhat hairy, but if your distro offers a package, go for it. It's beauty lies in the simple configuration. Want more remote threads? Alter a number in a text file.
The other function I really love it piping mail into scripts directly. Makes for cool auto-responders, or automated signatures/layouts/etcetera.

Sendmail is probably still the most widely used, though postfix is better on the resources and easier to configure.

At https://supportbee.com we are using to send and receive emails. However for sending emails, we are using mailgun and our postfix relays the emails to mailgun.