Email Server Suite

A complete private email server implementation in C++20 with POP3, IMAP, and SMTP support.

Features

POP3 Server (RFC 1939)
- Ports: 110 (plain), 995 (TLS)
- Commands: USER, PASS, STAT, LIST, RETR, DELE, NOOP, RSET, QUIT, TOP, UIDL, CAPA, STLS
- STARTTLS support
IMAP Server (RFC 3501)
- Ports: 143 (plain), 993 (TLS)
- Commands: LOGIN, LOGOUT, SELECT, EXAMINE, CREATE, DELETE, RENAME, LIST, STATUS, FETCH, STORE, COPY, SEARCH, EXPUNGE, UID, CAPABILITY, NOOP, STARTTLS
- Message flags and mailbox management
SMTP Server (RFC 5321)
- Ports: 25 (MTA), 587 (submission), 465 (TLS)
- Commands: HELO, EHLO, MAIL FROM, RCPT TO, DATA, RSET, NOOP, QUIT, VRFY, AUTH, STARTTLS
- AUTH PLAIN and AUTH LOGIN
- Local delivery and relay support
Common Features
- TLS/SSL encryption (STARTTLS and implicit TLS)
- SQLite-based user authentication
- Maildir storage format
- Configurable via INI-style config file
- Comprehensive logging

Requirements

C++20 compatible compiler (GCC 10+, Clang 12+)
CMake 3.20+
Boost 1.74+ (Asio)
OpenSSL 1.1.1+
SQLite 3.35+
Catch2 3.0+ (for tests, automatically fetched if not found)

Installing Dependencies

Ubuntu/Debian:

sudo apt update
sudo apt install build-essential cmake libboost-all-dev libssl-dev libsqlite3-dev

macOS (Homebrew):

brew install cmake boost openssl sqlite3

Fedora:

sudo dnf install gcc-c++ cmake boost-devel openssl-devel sqlite-devel

Building

# Clone the repository
git clone https://github.com/yourusername/email_server.git
cd email_server

# Create build directory
mkdir build && cd build

# Configure
cmake .. -DCMAKE_BUILD_TYPE=Release

# Build
cmake --build . -j$(nproc)

# Run tests
ctest --output-on-failure

# Install (optional)
sudo cmake --install .

Build Options

-DBUILD_TESTS=ON|OFF - Build test programs (default: ON)
-DBUILD_TOOLS=ON|OFF - Build utility tools (default: ON)
-DENABLE_TLS=ON|OFF - Enable TLS/SSL support (default: ON)
-DCMAKE_BUILD_TYPE=Debug|Release - Build type

Configuration

Copy the example configuration:

sudo mkdir -p /etc/email_server
sudo cp config/server.conf.example /etc/email_server/server.conf

Edit the configuration file:

sudo nano /etc/email_server/server.conf

Key settings to configure:

[tls] - SSL certificate and key paths
[database] - User database location
[storage] - Maildir root directory
[smtp] - SMTP server settings including hostname and local domains

SSL Certificates

Self-Signed (for testing)

./tools/generate_certs.sh mail.example.com /etc/ssl/email_server

Let's Encrypt (for production)

# Install certbot
sudo apt install certbot

# Obtain certificate
sudo certbot certonly --standalone -d mail.example.com

# Update configuration
# certificate = /etc/letsencrypt/live/mail.example.com/fullchain.pem
# private_key = /etc/letsencrypt/live/mail.example.com/privkey.pem

User Management

Use the create_user tool to manage users:

# Add a domain
./bin/create_user domain add example.com

# Add a user
./bin/create_user add user@example.com

# List users
./bin/create_user list

# Change password
./bin/create_user passwd user@example.com

# Show user info
./bin/create_user info user@example.com

# Delete user
./bin/create_user delete user@example.com

Running the Servers

Individual Servers

# SMTP server
./bin/smtp_server -c /etc/email_server/server.conf

# POP3 server
./bin/pop3_server -c /etc/email_server/server.conf

# IMAP server
./bin/imap_server -c /etc/email_server/server.conf

Systemd Services

Create systemd service files for production deployment:

/etc/systemd/system/email-smtp.service:

[Unit]
Description=Email SMTP Server
After=network.target

[Service]
Type=simple
ExecStart=/usr/local/bin/smtp_server -c /etc/email_server/server.conf
Restart=always
User=mail
Group=mail

[Install]
WantedBy=multi-user.target

Enable and start:

sudo systemctl enable email-smtp email-pop3 email-imap
sudo systemctl start email-smtp email-pop3 email-imap

DNS Configuration

Configure the following DNS records for your domain:

Type	Name	Value	Priority
A	mail.example.com	YOUR_IP	-
MX	example.com	mail.example.com	10
TXT	example.com	v=spf1 mx -all	-
PTR	YOUR_IP	mail.example.com	-

For SPF (prevents email spoofing):

v=spf1 mx -all

For DMARC (optional but recommended):

_dmarc.example.com TXT "v=DMARC1; p=quarantine; rua=mailto:admin@example.com"

Firewall Configuration

Open the required ports:

# UFW
sudo ufw allow 25/tcp    # SMTP
sudo ufw allow 465/tcp   # SMTPS
sudo ufw allow 587/tcp   # Submission
sudo ufw allow 110/tcp   # POP3
sudo ufw allow 995/tcp   # POP3S
sudo ufw allow 143/tcp   # IMAP
sudo ufw allow 993/tcp   # IMAPS

# iptables
sudo iptables -A INPUT -p tcp --dport 25 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 465 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 587 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 110 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 995 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 143 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 993 -j ACCEPT

Client Configuration

Thunderbird

Incoming Mail (IMAP):

Server: mail.example.com
Port: 993
Security: SSL/TLS
Authentication: Normal password

Incoming Mail (POP3):

Server: mail.example.com
Port: 995
Security: SSL/TLS
Authentication: Normal password

Outgoing Mail (SMTP):

Server: mail.example.com
Port: 587
Security: STARTTLS
Authentication: Normal password

Apple Mail

Add Account > Other Mail Account
Enter your name, email, and password
Configure manually:
- IMAP: mail.example.com:993 (SSL)
- SMTP: mail.example.com:587 (STARTTLS)

Outlook

File > Add Account
Manual setup > POP or IMAP
Configure:
- Account type: IMAP
- Incoming: mail.example.com:993 (SSL)
- Outgoing: mail.example.com:587 (TLS)

Testing

Manual Testing with telnet

SMTP:

telnet mail.example.com 25
EHLO client.example.com
AUTH PLAIN <base64-credentials>
MAIL FROM:<sender@example.com>
RCPT TO:<recipient@example.com>
DATA
Subject: Test
This is a test.
.
QUIT

POP3:

telnet mail.example.com 110
USER user@example.com
PASS password
STAT
LIST
RETR 1
QUIT

Testing with OpenSSL (TLS)

openssl s_client -connect mail.example.com:993
openssl s_client -connect mail.example.com:995
openssl s_client -starttls smtp -connect mail.example.com:587

Directory Structure

/var/mail/                    # Maildir root
  example.com/                # Domain
    username/                 # User maildir
      cur/                    # Read messages
      new/                    # New messages
      tmp/                    # Temporary
      .Sent/                  # Sent folder
      .Drafts/                # Drafts folder
      .Trash/                 # Trash folder

/var/lib/email_server/        # Server data
  users.db                    # User database

/etc/email_server/            # Configuration
  server.conf                 # Main config file

/var/log/email_server/        # Logs
  smtp.log
  pop3.log
  imap.log

Troubleshooting

Connection refused

Check if the server is running: systemctl status email-smtp
Check firewall rules: sudo ufw status
Verify ports are listening: ss -tlnp | grep -E '(25|110|143|465|587|993|995)'

Authentication failed

Verify user exists: ./bin/create_user info user@example.com
Check password: ./bin/create_user passwd user@example.com
Check logs: tail -f /var/log/email_server/smtp.log

TLS handshake failed

Verify certificate: openssl x509 -in /etc/ssl/email_server/server.crt -text
Check certificate chain: openssl verify -CAfile ca.crt server.crt
Ensure key matches certificate

Messages not delivered

Check maildir permissions: ls -la /var/mail/example.com/username/
Verify local_domains in config
Check SMTP logs for delivery errors

Security Recommendations

Always use TLS - Enable STARTTLS on all ports
Require authentication - Set require_auth = true for SMTP
Use strong passwords - Enforce via user management
Regular updates - Keep the system and dependencies updated
Firewall - Only open necessary ports
SPF/DKIM/DMARC - Configure email authentication
Monitoring - Set up log monitoring for suspicious activity
Backups - Regularly backup /var/mail and user database

License

MIT License - See LICENSE file for details.

Contributing

Contributions are welcome! Please submit pull requests or open issues on GitHub.

Name		Name	Last commit message	Last commit date
.github/workflows		.github/workflows
cmake		cmake
common		common
config		config
docker		docker
imap		imap
pop3		pop3
smtp		smtp
tests		tests
tools		tools
webmail		webmail
.env.example		.env.example
.gitignore		.gitignore
CLAUDE.md		CLAUDE.md
CMakeLists.txt		CMakeLists.txt
Dockerfile		Dockerfile
README.md		README.md
conanfile.py		conanfile.py
docker-compose.yml		docker-compose.yml

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