LPIC-1 108.3 — Mail Transfer Agent (MTA) Basics

Exam weight: 3 — LPIC-1 v5, Exam 102

What You Need to Know

From the official LPIC-1 objectives:

Create e-mail aliases.
Configure e-mail forwarding.
Knowledge of commonly available MTA programs (postfix, sendmail, exim).

Key files and commands: ~/.forward, /etc/aliases, sendmail, mailq, mail, postfix, sendmail, exim, newaliases.

Mail Concepts

A Mail Transfer Agent (MTA) routes email between servers using SMTP (Simple Mail Transfer Protocol) on port 25. When a message arrives, the MTA delivers it to the local inbox at /var/spool/mail/username.

DNS MX (Mail Exchanger) records direct mail to the correct server for a domain.

Common MTAs

MTA	Notes
Sendmail	The original Unix MTA; powerful but complex configuration
Postfix	Drop-in Sendmail replacement; most widely used today
Exim	Default MTA on Debian systems
qmail	Secure design; not commonly installed by default

Postfix provides a sendmail compatibility wrapper so scripts using sendmail continue to work without modification.

sendmail Command

The sendmail command (available on all MTAs as a compatibility interface) sends an email from the command line. The message body is read from stdin; terminate with a line containing only .:

$ sendmail user@example.com
Subject: Test

Hello, this is a test message.
.

Useful flags:

Flag	Description
`-bp`	List the mail queue (same as `mailq`)
`-q`	Flush the mail queue (attempt delivery of queued messages)
`-bi` / `-I`	Rebuild the aliases database (same as `newaliases`)

mail Command (MUA)

mail is a simple Mail User Agent for reading and sending mail from the terminal.

Send Mode

mail -s "Subject" recipient@example.com

The command reads the message body from stdin.

Read Mode

Running mail without arguments opens the inbox and shows a list of messages. Commands in read mode:

Command	Action
`N` (number)	Read message N
`d N`	Delete message N
`r N`	Reply to message N
`q`	Quit and save changes

Mail Queue

When a message cannot be delivered immediately, the MTA places it in the mail queue. Queue locations:

Sendmail: /var/spool/mqueue/
Postfix: /var/spool/postfix/

Queue Commands

mailq              # list queued messages
sendmail -bp       # same as mailq
sendmail -q        # force retry of all queued messages

/etc/aliases — System-Wide Aliases

/etc/aliases defines email aliases for the system. It maps alias names to one or more destinations:

# Format: alias: destination
postmaster:   root
root:         admin@example.com
helpdesk:     alice, bob, charlie
bugs:         /var/log/bugs.txt
complaints:   |/usr/local/bin/ticket-handler

Destination Types

Destination	Description
`username`	Local user
`user@domain`	Remote email address
`user1, user2`	Comma-separated list (distribution list)
`/path/to/file`	Append message to a file
`\|/path/to/command`	Pipe message body to a program
`:include:/path/to/file`	Read destinations from a file

After editing /etc/aliases, rebuild the binary alias database:

newaliases          # preferred command
sendmail -bi        # equivalent
sendmail -I         # equivalent

The compiled database is stored as /etc/aliases.db.

~/.forward — Per-User Forwarding

Each user can create a ~/.forward file to redirect their mail without requiring root access or a newaliases rebuild.

# Forward to another address
user@otherdomain.com

# Deliver locally AND forward
\username, user@otherdomain.com

# Pipe to a program
|/usr/bin/vacation

Rules for ~/.forward:

No newaliases needed — the MTA reads it directly.
The file must be owned by the user and not writable by others (group and world write bits must be clear).
The \username syntax with backslash delivers a copy locally, preventing forwarding loops.

Quick Reference

MTA = Mail Transfer Agent
  SMTP port 25
  Local inbox: /var/spool/mail/username
  Common MTAs: Postfix (most common), Sendmail (original),
               Exim (Debian default), qmail

sendmail:
  sendmail addr         send mail (body from stdin, end with .)
  sendmail -bp          list mail queue
  sendmail -q           flush/retry mail queue
  sendmail -bi / -I     rebuild aliases DB (= newaliases)

mail command:
  mail -s "Subject" addr    send mail (body from stdin)
  mail                      read inbox

Mail queue:
  mailq                 list queued messages
  sendmail -q           force retry

/etc/aliases:
  alias: destination(s)
  Destinations: username, user@domain, comma list,
                /file, |command, :include:/file
  After editing: run newaliases

~/.forward:
  user@domain           forward to address
  \username             keep local copy (prevents loop)
  |/path/cmd            pipe to program
  No newaliases needed; file must not be group/world writable

Exam Questions

What protocol does an MTA use to route email between servers? → SMTP on port 25.
Where is local user mail stored on the system? → /var/spool/mail/username
What DNS record type directs email to a mail server? → MX (Mail Exchanger) record.
Which MTA is the most widely used today and is a drop-in replacement for Sendmail? → Postfix.
Which MTA is the default on Debian-based systems? → Exim.
How do you send an email with sendmail from the command line? → Run sendmail user@domain, type the message body, then end with a line containing only ..
What does sendmail -bp do? → Lists the mail queue (same as mailq).
What does sendmail -q do? → Forces the MTA to retry delivery of all queued messages.
How do you send an email with a subject line using the mail command? → mail -s "Subject" recipient@example.com
What file defines system-wide email aliases? → /etc/aliases
What command must be run after editing /etc/aliases? → newaliases (or sendmail -bi or sendmail -I).
What file stores the compiled aliases database? → /etc/aliases.db
What does the destination |/path/to/script mean in /etc/aliases? → Pipe the message body to the specified program.
What is the purpose of ~/.forward? → Allows individual users to redirect or forward their incoming mail without root access.
Does ~/.forward require running newaliases after editing? → No, the MTA reads it directly.
What does \username mean in ~/.forward? → Deliver a local copy to that user (the backslash prevents forwarding loops).
What permission rule applies to ~/.forward? → The file must be owned by the user and must not be writable by group or others.
Where does Postfix store its mail queue? → /var/spool/postfix/

Exercises

Exercise 1 — System Alias

Add an alias so that mail sent to webmaster is delivered to both alice and bob@remote.example.com. What command rebuilds the database?

Answer

In /etc/aliases:

webmaster:   alice, bob@remote.example.com

Then run:

newaliases

Exercise 2 — File Delivery Alias

Create an alias bugs that appends incoming messages to /var/log/bugs.txt.

Answer

In /etc/aliases:

bugs:   /var/log/bugs.txt

Run newaliases after editing.

Exercise 3 — Per-User Forward

User carol wants all her incoming mail forwarded to carol@gmail.com while also keeping a local copy. Write the contents of her ~/.forward.

Answer

\carol, carol@gmail.com

The \carol part delivers locally; carol@gmail.com forwards externally. The backslash prevents an infinite forwarding loop.

Exercise 4 — Reading the Queue

How do you list all messages currently waiting in the mail queue?

Answer

mailq

or equivalently:

sendmail -bp

Exercise 5 — Sending from Command Line

Send a test message to root with the body “Disk check passed” using sendmail.

Answer

sendmail root <<EOF
Subject: Disk check

Disk check passed
.
EOF

Or interactively:

sendmail root
Subject: Disk check

Disk check passed
.

End the message with a line containing only ..

LPIC-1 Study Notes | Topic 108: Essential System Services