SMTP
(pronounced as separate letters) Short for Simple Mail Transfer Protocol, a protocol for sending e-mail messages between servers. Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP. In addition, SMTP is generally used to send messages from a mail client to a mail server. This is why you need to specify both the POP or IMAP server and the SMTP server when you configure your e-mail application.
POP
(1) Short for Post Office Protocol, a protocol used to retrieve e-mail from a mail server. Most e-mail applications (sometimes called an e-mail client) use the POP protocol, although some can use the newer IMAP (Internet Message Access Protocol).
There are two versions of POP. The first, called POP2, became a standard in the mid-80's and requires SMTP to send messages. The newer version, POP3, can be used with or without SMTP.
(2) Short for point of presence, an access point to the Internet. ISPs have typically multiple POPs. A point of presence is a physical location, either part of the facilities of a telecommunications provider that the ISP rents or a separate location from the telecommunications provider, that houses servers, routers, ATM switches and digital/analog call aggregators.
(3) Short for Programmed Operator (POP), a pseudo-opcode in a virtual machine language executed by an interpretive program. The Programmed Operator instructions provide the ability to define an instruction set for efficient encoding by calling subprograms into primary memory.
IMAP
Short for Internet Message Access Protocol, a protocol for retrieving e-mail messages. The latest version, IMAP4, is similar to POP3 but supports some additional features. For example, with IMAP4, you can search through your e-mail messages for keywords while the messages are still on mail server. You can then choose which messages to download to your machine.
IMAP was developed at Stanford University in 1986.
MUA
Short for mail user agent, the software that allows a user to access and manage e-mail, including reading, composing, disposing, printing and displaying e-mail messages. The MUA provides the interface between the user and the MTA. Eudora and Outlook are two popular MUAs.