1. Introduction

1.3.1. Incoming mail

The type of software that manages email on your computer falls into three loosely defined catagories: Mail Transport Agent, Mail Delivery Agent and Mail User Agent. There is an article describing generic mail flow. These mail agents are often incorporated into a single software application (eg evolution, mozilla mail, etc). Figure 1 shows a typical system for receiving email.

1. Mail Transport Agent (MTA), sometimes referred to as a Mail Transfer Agent. A mail transport agent is software that enables mail to travel from one mail system to another. Your provider's server uses an MTA to send you your mail. You must have an MTA at your end of the connection. A standard Debian installation uses exim, but you may come across others eg sendmail, smail, qmail, etc. Typically, the MTA passes the mail onto an MDA.

2. Mail Delivery Agent (MDA), sometimes referred to as a Local Delivery Agent (LDA). A mail delivery agent is software that takes mail from an MTA and passes it on to a mailbox or another MTA. A standard Debian installation uses procmail, although there are others eg deliver.

3. Mail User Agent (MUA). A mail user agent is the software that enables a user to write, send, receive and read email. A standard Debian installation uses mail for local mail. Other mail user agents include elm, pine, eudora, pegasus, etc.

Mail from your provider normally reaches you using the POP3 protocol. A protocol is a set of rules that govern how things communicate over the network (see theLinux dictionary).

1.3.2. Outgoing mail

Outgoing mail is sent to your provider's server, usually using the SMTP protocol. You use a mail user agent to write the message. A mail transport agent takes the message and passes it on to a mail transfer agent. The mail transport agent passes the message to a mail transport agent on your provider's server (see figure 2).

1.4.1. Debian local mail

If you have a fairly standard Debian installation, your system is probably set up like this. exim is the MTA and mail is the MUA. mail writes to the mailbox local-user-1@local-host. The contents of the mailbox are stored in the file /var/mail/local-user-1 (sometimes called the spool file). Here local-user-1 is the name that you use to login to your computer.

A user can read the mail using mail as the MUA. The command is:

mail
    

The user (local-user-1) can write a mail message to another user (local-user-2) on the system using the command:

mail local-user-2
    

1.4.2. Debian external mail

You probably manage your outgoing mail with an email application. This document focuses on evolution, but you may come across others eg links (text-based), lynx (text-based), Mozilla Mail and kmail. These email clients are designed to carry out all the necessary functions (MTA, MDA, and MUA) in one application.

In fact, evolution is not just an email client. It is a graphical groupware suite and also includes a calendar, task manager and contacts database. evolution stores the contents of its email inbox in the user's home directory (˜/evolution/local/Inbox).

1.4.3. Email filter

One reason for concentrating on evolution is that it has good, built-in functions for filtering mail. You can easily set up a filter to place all mail with a particular header into a particular folder. This approach means that you must first add the header to the original message before it reachesevolution. The task of identifying a spam email and adding identifying headers to the message is handled by spamassassin and procmail.

1.4.4. Example of a spam email modified using spamassassin

X-Spam-Status: Yes, hits=5.7 required=5.0
     tests=VIAGRA,BASE64_ENC_TEXT,SUBJ_ALL_CAPS version=2.20
X-Spam-Flag: YES
X-Spam-Level: *****
X-Spam-Checker-Version: SpamAssassin 2.20 
     (devel $Id: x27.html.en,v 1.1 2004/04/30 21:50:38 chrislale Exp $)
X-Spam-Prev-Content-Type: text/plain
X-Spam-Prev-Content-Transfer-Encoding: base64
X-Evolution-Source: mbox:/var/mail/local-user
     

Subject: *****SPAM***** WEEKEND VIAGRA
     

SPAM: -------------------- Start SpamAssassin results ----------------------
SPAM: This mail is probably spam.  The original message has been altered
SPAM: so you can recognise or block similar unwanted mail in future.
SPAM: See http://spamassassin.org/tag/ for more details.
SPAM: 
SPAM: Content analysis details:   (5.7 hits, 5 required)
SPAM: Hit! (2.4 points)  BODY: Plugs Viagra
SPAM: Hit! (1.4 points)  Message text disguised using base-64 encoding
SPAM: Hit! (1.9 points)  Subject is all capitals
SPAM: 
SPAM: -------------------- End of SpamAssassin results ---------------------
     

1.7.1. Names used on your (local) computer

1.7.2. Names used on your provider's (remote) server