Howto Tutorial for a secure linux mail server.
Tutorial has to be step by step howto with explanations of steps
Debian lstest stable
Full Mail Server Solution w/ Virtual Domains & Users (Debian , Postfix latest stable, webmin latest ,postfixadmin, MySQL, DoveCot,spamassassine, DSpam, ClamAV,mailscanner, mail watch, Postgrey, RBL, webmail)
I case you have better suggestion of software chosen above please give your suggestion as well with some explanation why.
Recommend you run postfix with mySQl for alias screening and ClamAV/Spamassassin, and with a set of security rules for spam blocking. I did this for my company and supported in the last 3 yrs with much success. Have a set of proven rules for you.
You can setup encryption for your email also if that is needed.