Mastering Sendmail Configuration in Linux

Complete Guide to Mail Server Configuration, SMTP, POP3/IMAP, Architecture, Security, and Spam Protection (Production Ready)

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📌 What This Article Covers

This guide provides a complete and beginner-friendly deep dive into Sendmail, one of the most widely used Mail Transfer Agents (MTA) in Linux systems.

You will learn:

• How Sendmail works internally
• Key configuration files and their purpose
• Mail flow and queue processing
• SMTP configuration and testing
• POP3/IMAP setup using Dovecot
• Mail routing, aliases, and access control
• Spam filtering using DNSBL and SpamAssassin
• Real-world production architecture

This is a practical, production-oriented guide for Linux administrators and DevOps engineers.

🎯 Goal of This Guide

By the end of this article, you will be able to:

• Understand Sendmail architecture and workflow
• Configure Sendmail to send and receive emails
• Enable external email communication
• Implement access control and security
• Set up POP3/IMAP services
• Configure spam filtering mechanisms
• Debug and monitor mail flow

Sendmail Architecture Overview

Real-World Mail Flow

1. User sends mail → Local queue (/var/spool/clientmqueue)
2. Sendmail daemon accepts SMTP connections (Port 25)
3. Queue runner processes pending emails
4. Mail delivered to:

• Local mailbox (/var/spool/mail/user)
• Remote mail servers via SMTP

5. POP3/IMAP (Dovecot) allows users to read emails

What is Sendmail?

Sendmail is a monolithic Mail Transfer Agent (MTA).

It handles:

• Receiving emails (SMTP)
• Routing emails
• Routing messages between servers
• Delivering emails locally or remotely
• Managing mail queues

It works closely with:

• Procmail → Local mail delivery
• Dovecot → POP3/IMAP access
• SpamAssassin → Spam filtering

Core Sendmail Components

📁 Important Configuration Files

| ---------------------------- | ----------------------------- |
| File                         | Purpose                       |
| ---------------------------- | ----------------------------- |
|  /etc/mail/sendmail.mc       | Main macro configuration file |
|  /etc/mail/sendmail.cf       | Compiled configuration file   |
|  /etc/mail/access            | Access control rules          |
|  /etc/mail/access.db         | Compiled DB file              |
|  /etc/mail/local-host-names  | Local domains                 |
|  /etc/mail/aliases           | Mail alias mapping            |
|  /var/spool/mail/            | User mailboxes                |
|  /var/spool/clientmqueue     | Mail queue                    |
|  /etc/mail/virtusertable     | Virtual domain mapping        |
|  /etc/mail/helpfile          | SMTP help commands            |
| ---------------------------- | ----------------------------- |

Sendmail uses m4 macro language to simplify configuration.

Mail Storage and Queues

📂 Mailbox Storage

/var/spool/mail/

Each user has a mailbox:

/var/spool/mail/username

This uses the traditional Unix mbox format.

📂 Mail Queue

/var/spool/clientmqueue

• Stores outgoing mail
• Processed by queue runner daemon
• Runs every minute

Sendmail Daemons

Sendmail runs two main processes:

1. SMTP Listener

• Accepts connections on port 25

2. Queue Runner

• Processes queued emails periodically

This ensures reliable email delivery even during failures.

Sendmail Configuration (Step-by-Step)

By default, Sendmail listens only on localhost.

To allow network access:

Step 1: Enable Sendmail to Accept External Mail

vi /etc/mail/sendmail.mc

Find and comment this line:

dnl DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

Step 2: Generate Configuration File

cd /etc/mail
make

or

m4 sendmail.mc > sendmail.cf

Step 3: Restart Sendmail Service

service sendmail restart

Now Sendmail accepts external emails.

Step 4: Verify Port 25

Test SMTP manually:

telnet localhost 25
netstat -antp | grep :25

Expected OutPut:

0.0.0.0:25

Mail Testing (SMTP) Commands:

telnet localhost 25
helo domain.com
mail from:<user@domain.com>
rcpt to:<receiver@domain.com>
data
Test mail - message content
.
quit

Mail Aliases Configuration

vi /etc/aliases

Example:

user5: user2

Apply changes:

newaliases

Configure Mail Relay (Access Control — ACL)

Control who can send mail:

vi /etc/mail/access

Example:

@domain.com RELAY
192.168.10 RELAY
@spamdomain.com REJECT

Update DB:

postmap /etc/mail/access

Restart:

service sendmail restart

Configure Local Domains

vi /etc/mail/local-host-names

Add:

@domain.com

Configure POP3/IMAP (Dovecot)

Install Dovecot and configure:

yum install dovecot

vi /etc/dovecot.conf    # Edit

protocols = pop3 imap   # Enable protocols:

service dovecot restart # Start service:

chkconfig dovecot on    #enable service 

Test POP3

telnet localhost 110
user username
pass password

Virtual Domains Configuration

vi /etc/mail/virtusertable # Edit

@domain.com localuser      # Example:  This maps domain emails to local users.

service sendmail restart   # Restart Sendmail

Enable SSL for IMAP (IMAPS)

Generate certificate:

cd /usr/share/ssl/certs
make dovecot.pem

Provide details:

• Country: IN
• State: Maharashtra
• City: Mumbai
• Common Name: your-domain

Enable Secure Protocols

vi /etc/dovecot.conf

protocols = imap imaps pop3 pop3s

Restart:

service dovecot restart

Spam Protection in Sendmail

DNS Blacklist (DNSBL)

Add in sendmail.mc:

FEATURE(`dnsbl', `bl.spamcop.net')
FEATURE(`dnsbl', `sbl.spamhaus.org')

Rebuild config:

make
service sendmail restart

SpamAssassin Setup

Install

yum install spamassassin

Enable Service

chkconfig spamassassin on
service spamassassin start

Configure Procmail

cp /etc/mail/spamassassin/spamassassin-spamc.rc /etc/procmailrc

Edit Config

vi /etc/mail/spamassassin/local.cf

Key settings:

required_hits 5.0
rewrite_subject 1
subject_tag *****SPAM*****
use_bayes 1
auto_learn 1

Key Points:

• Spam threshold
• Subject tagging
• Bayesian filtering
• Language filtering

Validate Config

spamassassin --lint

Monitoring and Debugging

Useful commands:

mail -v user
mailq
sendmail -q
tail -f /var/log/maillog

Check open ports:

netstat -antp | grep :25

Test Configuration

spamassassin --lint

🙌 Thank You..! 😊👏

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