Throughout this document we will be using a number of acronyms to describe things. It will help you (if you've never heard of these before) to know them in advance because they will be used quite often in this document without elaboration. Here is the short list with their associated definitions.
MTA - Mail Transfer Agent. Accepts mail from other MTAs and mail users (you and I).
MDA - Mail Delivery Agent. Accepts inbound mail from an MTA and delivers it to the appropriate user on the local machine.
MUA - Mail User Agent. Software used by humans to download mail, upload to an MTA, create, and read mail.
POP3 - Post Office Protocol. The third version of this protocol POP3 allows a client computer to retrieve electronic mail from a POP3 server via a (temporary) TCP/IP or other connection. It does not provide for sending mail, which is assumed to be done via SMTP or some other method.
IMAP - the Internet Message Access Protocol - is a method of accessing electronic messages kept on a (possibly shared) mail server.
SMTP - Simple Mail Transport Protocol. A server to server protocol, so other protocols (POP3, IMAP etc.) are used to access the messages. The SMTP dialog usually happens in the background under the control of the message transport system, e.g. sendmail but it is possible to interact with an SMTP server using telnet to connect to the normal SMTP port, 25. E.g.
telnet mhs-relay.ac.uk 25 You should normally start by identifying the local host: HELO wombat.doc.ic.ac.uk
DNS - Domain Name Service aka bind.
UCE - Unsolicited Commercial Email a.k.a spam