Here's a short list of the abbreviations used in the following sections and in the security vendor matrix (shown at the end of the article):
ACL: Access control list
DIY: Do it yourself
DMZ: Demilitarized zone. A DMZ is used in server firewall setups where a computer is located outside the firewall but is reachable by the internal network. Systems in the DMZ can send and receive to/from the entire Internet.
MTA: Mail Transport AgentSendmail, Postfix, Microsoft Exchange, Qmail