To give you a general idea of my setup process, based on a static IP:
- Repurpose a known, secure distribution of Linux.
- Install Webmin for easier configuration
- Replace Sendmail with Postfix
- Open required ports on server
- Redirect ports on separate firewalll/router to email server
- Edit DNS settings to provide global MX records
- Setup separate virtual host with Postfix for MX Backup
Redundancy is the prime consideration: if your mail server is down or inaccessible, your mail will bounce and can be lost forever.
As
Saab Dastard said, there is a lot more to configure for security, that's if your ISP permits traffic through the relevant ports.
Depending on your needs, you may find that a mail host/hub is what you really want? A Dovecot server that can store all your mail for ready access, but cannot send mail. This removes most of the security issues.
Good luck.