1.3 Definitions

Before going any further, let's define some terms used when discussing email services. The Mail Transfer Agent (MTA) is responsible for moving email between different email servers using the Simple Mail Transfer Protocol (SMTP), defined by RFC 2821 [KLE01]. Examples of UNIX MTAs include sendmail, postfix [POS], Exim [EXI], and qmail [QMA]. MTAs temporarily store email that they have accepted but not yet delivered in the message queue (or mail queue, or just queue). The Local Delivery Agent (LDA) is the program that receives email from an MTA and places it in the message store or mail spool. The message store is a data repository where email messages reside, typically sorted by user. A user queries this repository using a Mail User Agent (MUA) such as Eudora or Netscape. The query is performed via one of several mechanisms, such as the Post Office Protocol (POP), which is the service provided by most ISPs, or Internet Message Access Protocol (IMAP), which is commonly used by many businesses. A program such as mailx or elm may also be referred to as an MUA, but these applications access the message store via the filesystem on the same server on which the email is stored. Common Open Source POP daemons used on email servers include Qpopper [QPO] and CUCIpop [CUC], and version 3 of POP itself is defined in RFC 1939 [MR96]. The three most common Open Source IMAP servers are Cyrus [CYR], UW-IMAP [UWI], and the Courier IMAP [COU] packages. Version 4r1 of IMAP is defined in RFC 2060 [CRI96]. Some organizations allow email access via a World Wide Web (WWW) interface. Often called Web Mail solutions, these systems won't be covered in depth in this book. A good overview on obtaining, building, installing, and running Open Source IMAP and Web mail services is available in Managing IMAP, by Dianna and Kevin Mullet [MM00]. A good overview of the email protocols mentioned here is found in David Wood's book, Programming Internet Email [WOO99].