Mail -- SMTP, POP, and IMAP (Unix Power Tools, 3rd Edition)

46.8. Mail -- SMTP, POP, and IMAP

Email is one of the most well-known and commonly used Internet services. The core of Internet email is the Simple Message Transfer Protocol (SMTP), which defines a simple, extensible mechanism by which hosts can exchange mail messages. SMTP is spoken by programs known as Message Transfer Agents (MTAs); sendmail is the most well known of these and is included with the vast majority of Unixes. qmail , postfix, and exim are other common MTAs (I use qmail on all of my systems). Configuring an MTA generally involves telling it your default domain name for outgoing email, setting up whether it allows relaying and if so, under what limits (see below), possibly setting up spam filtering, and the like. It may also involve setting up MX records (Section 46.9) for your domain(s).

Relaying is when an MTA allows someone to connect and send an email to an email address not served by that MTA. If you want to allow someone on your local machine or local subnet to send outgoing email via your MTA, this is a very good thing. An open relay allows anyone to send outgoing email, and this allows spammers to use your machine to send their spam. As you might guess, this is a Very Bad Thing. All MTAs have ways of limiting relaying so that local users can send email but spammers can't use your machine. Check your MTA's documentation, or take a peek at http://www.mail-abuse.org for more information.

Mail User Agents (MUAs or just UAs) provide the interface between users and MTAs. On Unix, these include programs such as mail, mailx, elm, and mutt, all of which work directly with the filesystem. Webmail clients are also MUAs, but they run under a webserver to provide networked access to mail. Often, though, you want to be able to use a MUA on another workstation that may or may not be a Unix machine, in which case you need some sort of MUA proxy to manage the mail and communicate with the remote MUA.

Post Office Protocol (POP or POP3) and Internet Message Access Protocol (IMAP) are two different ways of providing access to remote MUAs. POP is focused on retrieving messages from a mail server and having the MUA store them, where IMAP is focused on managing mail on a mail server remotely rather than copying it to the client machine. Freely available POP servers include qmail-pop3d (which comes with qmail) and qpopper (the Berkeley POP3 server, now maintained by Qualcomm), along with a wide variety of others, depending what you're looking for. Freely available IMAP servers include courier-imap and the University of Washington IMAP server (imap-uw).

-- DJPH