I really like the Postfix smtp server. It is incredibly flexible yet manageable as well.
If I understand correctly, Postfix was created as a replacement for the sendmail binary, and has evolved into so much more. The logo of Postfix is a cartoon mouse holding a bag of mail while running.
I first learned about Postfix from a Gentoo "Howto" document, and since then I haven't looked back. I regularly use it as a backup MX, delivering mail, relaying mail, sending mail, and filtering spam with dnsrbls.
Postfix integrates quite well with dovecot.