Is this enough for a working mail server?

view story

http://unix.stackexchange.com – I have an internet connection with a public, static IPv4 address (the IP address has a PTR record for foo.org domain, and I own foo.org domain, and have nameservers for it elsewhere, mx record is set to use the static ip). I have an OpenBSD i386 installed on a machine that is the only thing that uses this internet connection. If I nmap all the port of it from the internet side, I can't see any open ports. Great. Since no service is running on the OpenBSD machine that is faced to the internet. I need to install a mail server on it, so people could use it for sending/receiving e-mails secure (HowTos)