Email Protocols

Email relies on three main protocols for sending and receiving messages.

SMTP (Simple Mail Transfer Protocol)

Handles outgoing email

Basics

Port 25 (unencrypted)
Port 587 (TLS)
Port 465 (SSL/TLS)
Push protocol (client to server)
Used for sending email

How SMTP Works

1. Client connects to SMTP server
2. MAIL FROM: Sender address
3. RCPT TO: Recipient address
4. DATA: Message content
5. . (period): End of message
6. QUIT: Close connection

SMTP Commands

# Connect to SMTP server
telnet mail.example.com 25

# Commands
HELO mail.example.com
MAIL FROM:<sender@example.com>
RCPT TO:<recipient@example.com>
DATA
Subject: Test

This is a test email.
.
QUIT

POP3 (Post Office Protocol v3)

Handles incoming email

Basics

Port 110 (unencrypted)
Port 995 (SSL/TLS)
Pull protocol (server to client)
Downloads email to client
Typically deletes from server

How POP3 Works

1. Client connects to POP3 server
2. USER: Provide username
3. PASS: Provide password
4. STAT: Check message count
5. RETR: Retrieve message
6. DELE: Delete message
7. QUIT: Close connection

IMAP (Internet Message Access Protocol)

Advanced incoming email protocol

Basics

Port 143 (unencrypted)
Port 993 (SSL/TLS)
Pull protocol (server to client)
Keeps email on server
Supports folders and flags
Preferred for modern clients

Why IMAP?

Access mail from multiple devices
Folder support
Flag/tag support
Partial download
Server-side search

POP3 vs IMAP

POP3              IMAP
Port 110/995      Port 143/993
Download only     Sync
Local storage     Server storage
Simple            Complex
Basic features    Advanced features
Good for mobile   Better for desktop

Email Security

SMTP TLS/SSL

# Port 587 (STARTTLS)
# Port 465 (Implicit SSL)

# Configure in postfix
# /etc/postfix/main.cf
smtp_tls_security_level = encrypt
smtp_use_tls = yes

SPF (Sender Policy Framework)

DNS record specifying authorized mail servers

v=spf1 include:_spf.example.com ~all

DKIM (DomainKeys Identified Mail)

Digitally signs emails

DMARC (Domain-based Message Authentication)

Policy enforcement for SPF and DKIM

v=DMARC1; p=reject; rua=mailto:admin@example.com

Email Server Setup

Postfix (SMTP)

# Install
sudo apt install postfix

# Configuration
/etc/postfix/main.cf

# Start
sudo systemctl start postfix

Dovecot (POP3/IMAP)

# Install
sudo apt install dovecot-core dovecot-imapd dovecot-pop3d

# Configuration
/etc/dovecot/dovecot.conf

# Start
sudo systemctl start dovecot

Testing Email

# Send test email via SMTP
sendmail user@example.com << EOF
Subject: Test

Test message
EOF

# Check mail queue
postqueue -p

# Flush queue
postqueue -f

# Check logs
sudo tail -f /var/log/mail.log

Common Email Ports

25 - SMTP (unencrypted)
110 - POP3 (unencrypted)
143 - IMAP (unencrypted)
465 - SMTPS (SSL)
587 - SMTP TLS
993 - IMAPS (SSL)
995 - POP3S (SSL)

Best Practices

Use TLS/SSL encryption
Implement SPF, DKIM, DMARC
Monitor email queue
Keep email software updated
Implement authentication
Monitor for abuse
Use strong passwords
Backup email data
Implement rate limiting
Monitor for spam and phishing

Email Protocols

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