The SMTP is a venerable protocol for sending and receiving e-mails. The entire Internet depends on this simple, yet very robust protocol. If you're using e-mails in any way, you're bound to be using SMTP at some point (unless you're stuck with some proprietary solutions).
Software which deals with transfer of e-mails between different computers is usually referred to as Mail Transfer Agent (MTA for short). There's quite a number of different MTA implementations out there. This guide will deal with the following: