🏠 Home 📥 Installation ⚙️ Configuration 🌍 Environment 🐳 Docker 📧 Postfix 📦 Releases 🐳 Images 📄 License

Postfix Integration

Learn how to integrate GoSMTP-dup with Postfix for seamless email duplication.

Overview

Postfix can route specific domains or all emails through the SMTP duplicator using transport maps. This allows you to duplicate emails without changing your existing mail flow.

Configuration Steps

Step 1: Configure the SMTP Duplicator

First, configure GoSMTP-dup to listen and relay emails:

# config.yaml
smtp:
  listen: "127.0.0.1:2525"
  domain: "mail.example.com"
relay:
  destination_primary: "oldmail.example.com:25"    # Current mail server
  destination_backups:
    - "newmail.example.com:25"                      # New mail server (migration target)
    - "archive.example.com:25"                      # Archive server

Step 2: Add Transport to master.cf

You have two options for running the duplicator:

Let Postfix manage the duplicator as a permanent service:

# SMTP duplicator permanent service
127.0.0.1:2525  inet  n       -       y       -       1       /usr/local/bin/gosmtp-dup
    user=postfix directory=/etc/smtp-dup

Note: Runs permanently as an inet service managed by Postfix. The proxy speaks SMTP natively, so Postfix can communicate with it directly.

Option B: External Service

For external service, no changes needed in /etc/postfix/master.cf. Postfix will use the default SMTP transport to connect to the external service.

Step 3: Configure Transport Maps

In your /etc/postfix/main.cf, add or update:

transport_maps = hash:/etc/postfix/transport

Step 4: Create Transport File

Create /etc/postfix/transport to define which emails go through the duplicator:

For both options (same configuration):

# Route specific domains through the duplicator
example.com     [127.0.0.1]:2525
.example.com    [127.0.0.1]:2525
test.com        [127.0.0.1]:2525

# Route all outbound email through duplicator
*               [127.0.0.1]:2525

# Route specific recipients
user@example.com        [127.0.0.1]:2525
admin@example.com       [127.0.0.1]:2525

Step 5: Update Transport Map and Reload

# Compile the transport map
postmap /etc/postfix/transport

# For Option B: Start external service
systemctl start gosmtp-dup
systemctl enable gosmtp-dup

# Reload Postfix configuration
systemctl reload postfix

# Verify configuration
postfix check

Note: For Option A (Postfix-managed), the duplicator process will be started automatically by Postfix when needed.

Testing the Integration

Test Email Flow

# Send a test email
echo "Test email" | mail -s "Test Subject" user@example.com

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

# Check duplicator logs
journalctl -u gosmtp-dup -f

Verify Transport

# Check if transport is applied
postmap -q example.com /etc/postfix/transport

# Test transport lookup
echo "example.com" | postmap -q - /etc/postfix/transport

Advanced Configurations

Conditional Routing

Route only certain email types through the duplicator:

# In transport file
newsletter@example.com  smtp-dup:[127.0.0.1]:2525
alerts@example.com      smtp-dup:[127.0.0.1]:2525

Multiple Duplicators

Use different duplicators for different purposes:

# Different duplicators for different domains
internal.com    smtp-dup:[127.0.0.1]:2525
external.com    smtp-dup2:[127.0.0.1]:2526

Then add another transport in master.cf:

smtp-dup2   unix  -       -       n       -       -       smtp
    -o smtp_generic_maps=
    -o smtp_destination_concurrency_limit=2

Fallback Configuration

Configure fallback if duplicator is unavailable:

# In main.cf
fallback_transport = smtp
smtp-dup_destination_recipient_limit = 1
smtp-dup_destination_concurrency_limit = 1

Migration Scenarios

Scenario 1: Gradual Migration

# Phase 1: Test with limited domains
# transport:
test.example.com    smtp-dup:[127.0.0.1]:2525

# duplicator config:
relay:
  destination_primary: "oldmail.example.com:25"
  destination_backups:
    - "newmail.example.com:25"

# Phase 2: Migrate critical domains
# transport:
example.com         smtp-dup:[127.0.0.1]:2525
.example.com        smtp-dup:[127.0.0.1]:2525

# duplicator config stays the same

# Phase 3: Switch primary destination
# transport stays the same

# duplicator config:
relay:
  destination_primary: "newmail.example.com:25"    # Switched!
  destination_backups:
    - "oldmail.example.com:25"                      # Now backup

Scenario 2: A/B Testing

# Split traffic between old and new servers
relay:
  destination_primary: "oldmail.example.com:25"
  destination_backups:
    - "newmail.example.com:25"
    - "monitoring.example.com:25"

Monitoring and Logs

Postfix Logs

# Monitor Postfix routing decisions
tail -f /var/log/mail.log | grep "transport=smtp-dup"

# Check for transport errors
grep "smtp-dup" /var/log/mail.log | grep "error"

Duplicator Logs

# Monitor duplicator activity
journalctl -u gosmtp-dup -f

# Check for relay failures
journalctl -u gosmtp-dup | grep "error"

Troubleshooting

Common Issues

  1. Transport not applied: 
    # Verify transport map compilation
postmap -q example.com /etc/postfix/transport

# Check main.cf configuration
postconf transport_maps
  2. Connection refused: 
    # Check if duplicator is listening
netstat -tulpn | grep 2525

# Test connection manually
telnet 127.0.0.1 2525
  3. Emails not duplicated: 
    # Check duplicator configuration
journalctl -u gosmtp-dup | grep "destination_primary"

# Verify backup servers are reachable
telnet backup.example.com 25

Debugging Commands

# For Option A (Postfix permanent service):
# Check if service is running
netstat -tulpn | grep 2525
ps aux | grep gosmtp-dup

# For Option B (External Service):
# Test SMTP connection to duplicator
echo -e "EHLO test\nQUIT" | nc 127.0.0.1 2525

# General debugging:
# Check Postfix configuration syntax
postfix check

# View active transport maps
postconf -d | grep transport_maps

# Test email routing
echo "test@example.com" | postmap -q - /etc/postfix/transport

Performance Considerations

Option A (Postfix permanent service)

Option B (External Service)

← Back: Docker Usage Next: Home →