e-mail system

A complete e-mail system can consist of all the following components¹:

Mail Storage Agent (MSA)

eg: qpopper

Mail Retrieval Agent (MRA)

eg: fetchmail, getmail

vacation

Mail Delivery Agent (MDA)

eg: procmail, maildrop

biff

eg: kbiff, gbiff, ixbiff, xbiff, wmbiff

Mail User Agent (MUA)

eg: mutt, pine, balsa, kmail, evolution, tkrat, elm, mush, mh

Pager

eg: less, more

Editor

eg: vim, emacs

OpenPGP Client²

eg: PGP, GnuPG

Signature Generator

eg: signature, siggen

Mail Transfer Agent (MTA)

eg: exim, sendmail, qmail, smail

List Server

eg: listserv, majordomo

How these can fit together:

1. My ISP's MSA holds e-mail sent to me.
2. My MRA gets it from my ISP (using POP or IMAP).
3. My vacation program could send an automated reply to all incoming e-mail (or just some).
4. My MDA organises and filters my incoming e-mail.
5. My biff lets me know that new e-mail has arrived.
6. My MUA shows me a list of e-mail and allows me to deal with each appropriately by either reading:
   1. My OpenPGP client can decrypt e-mail or verify signatures.
   2. My pager allows me to read through an e-mail.
   ...or writing:
   1. My text editor (or word processor) allows me to write the e-mail.
   2. My signature generator adds a tagline.
   3. My OpenPGP client encrypts or signs the e-mail.
7. My MUA passes the e-mail to my MTA which passes it to my ISP's MTA (using SMTP).
8. If my e-mail was addressed to a mailing list, a list server will expand the address.
9. My e-mail is received by someone else's ISP; GOTO 1.

This is an extreme example of a system that uses all possible components. The only required components are:

• MSA
• MRA
• MUA
• Pager
• Editor
• MTA

Quite often (moreso in Windows than Unix) some of these components are combined into a single program. For example, MUAs can often retrive and transfer e-mail themselves and include an internal pager, editor, OpenPGP client, and signature generator. There are also some servers that act as a MSA, MTA, and list server.

1: All examples are taken from Unix (where these components are more seperable) and I tried to include some of the most popular options. Please give additional examples under the specific node unless you are providing an example of a complete system.

2: In theory you could use a standard other than OpenPGP (even other than PKI!) to encrypt and sign your e-mail, but I've never heard of someone doing such a thing.