Setting Up SMTP with Postfix on Linux: A Step-by-Step Guide

Introduction
SMTP (Simple Mail Transfer Protocol) is essential for sending emails from applications or servers. Postfix is a reliable, secure, and easy-to-configure mail transfer agent (MTA) for Linux. This guide walks you through setting up a basic SMTP server using Postfix for outbound emails. Note: This setup is ideal for transactional emails (e.g., notifications) — not for running a full mail server.

Prerequisites

1. A Linux server (Ubuntu/Debian/CentOS)
2. sudo privileges
3. A domain name pointing to your server’s IP (e.g., yourdomain.com)
4. Port 25 open in your firewall (check with your cloud provider; some block it by default).

Step 1: Install Postfix

Update packages and install Postfix:

# Ubuntu/Debian
sudo apt update
sudo apt install postfix -y

# CentOS/RHEL
sudo yum install postfix -y
sudo systemctl enable postfix

During installation:

• Select Internet Site when prompted.
• Enter your server’s domain (e.g., yourdomain.com).

Step 2: Configure Postfix

Edit the main configuration file:

sudo nano /etc/postfix/main.cf

Update these key settings:

# Set your domain
myhostname = mail.yourdomain.com
mydomain = yourdomain.com
myorigin = $mydomain

# Limit to local network/IPs sending mail
inet_interfaces = all
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 YOUR_SERVER_IP

# Security (adjust for IPv6 if needed)
inet_protocols = ipv4
smtpd_tls_security_level = may

Replace YOUR_SERVER_IP with your public IP (e.g., 192.0.2.1). Use ip a to find it.

Save the file and restart Postfix:

sudo systemctl restart postfix

Step 3: Test Your SMTP Server

Test 1: Local Command

Use telnet to connect:

telnet localhost 25

After connecting, type:

ehlo localhost
mail from: test@yourdomain.com
rcpt to: your-personal-email@gmail.com
data
Subject: Test Email
Hello, this is a test!
.
quit

Check your Gmail’s Spam folder for the test email.

Test 2: Send via mail Command

Install mailutils:

sudo apt install mailutils -y  # or sudo yum install mailx

Send a test email:

echo "Test body" | mail -s "Test Subject" your-email@gmail.com

Step 4: Secure Your Server

Firewall Rules

Allow port 25:

sudo ufw allow 25  # Ubuntu
# or
sudo firewall-cmd --permanent --add-port=25/tcp && sudo firewall-cmd --reload  # CentOS

Prevent Spam

• Restrict Relay: Ensure mynetworks only includes trusted IPs.
• Enable TLS (Optional): Add to main.cf:

  smtpd_use_tls = yes
  smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
  smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key

Troubleshooting

1. Check Logs:

   tail -f /var/log/mail.log  # Ubuntu/Debian
   tail -f /var/log/maillog   # CentOS

2. Verify Configuration:

   sudo postfix check

3. Test SMTP Connectivity Externally:

   telnet yourdomain.com 25

   (If blocked, check firewall/ISP restrictions.)

Conclusion

You’ve now set up a basic SMTP server with Postfix! This allows your applications to send emails directly from your Linux server. For production:

• Use a dedicated email service (e.g., SendGrid, Mailgun) to avoid deliverability issues.
• Configure SPF/DKIM/DMARC records in DNS to prevent emails from being marked as spam.
• Monitor logs regularly for abuse attempts.

> Next Steps:
> – Encrypt emails with TLS: Let’s Encrypt Certbot Guide
> – Configure spam filtering: sudo apt install spamassassin

Questions? Share them in the comments below! 🐧✉️