When your primary mail server goes down, Postfix automatically queues messages on your backup server. By default, Postfix follows these retry intervals:

Initial delay: 300 seconds (5 minutes)
Maximal delay: 4000 seconds (~1.1 hours)
Backoff scheme: Doubles after each attempt

Method 1: Using postqueue command

# Flush entire queue:
sudo postqueue -f

# Flush specific queue ID:
sudo postqueue -i 7D9B533002A

Method 2: Using sendmail command

# Process all deferred mail:
sudo sendmail -q

# Process specific queue:
sudo sendmail -q -v deferred

# View queue contents:
postqueue -p

# Check deferred queue specifically:
mailq | grep deferred

For permanent configuration changes, edit /etc/postfix/main.cf:

# Reduce initial retry delay
minimal_backoff_time = 60s

# Set maximum retry attempts
maximal_queue_lifetime = 5d

# Increase simultaneous delivery attempts
default_destination_concurrency_limit = 20

Create a cron job for periodic flushing:

# Add to crontab -e
*/5 * * * * /usr/sbin/postqueue -f

Or use a more sophisticated script that checks primary server availability first:

#!/bin/bash
PRIMARY_SERVER="mail.example.com"
if ping -c 1 $PRIMARY_SERVER &> /dev/null
then
    /usr/sbin/postqueue -f
fi

If messages still won't deliver:

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

# Verify Postfix service status:
postfix status

# Test DNS resolution for target domain:
dig MX example.com

When Postfix encounters delivery failures (like when your primary MX server is down), it implements an exponential backoff algorithm. This means:

• First retry after ~15 minutes
• Second retry after ~30 minutes
• Subsequent retries with increasing intervals

The most effective way to force immediate retry attempts is:

sudo postqueue -f

This signals Postfix to attempt delivery of all queued messages immediately, bypassing the scheduled retry times.

For more granular control:

# Process only deferred queue
sudo postqueue -p | grep deferred | awk '{ print $1 }' | postsuper -r -

# Process specific message by queue ID
sudo postcat -q ABC123DEF456 | postfix sendmail -G -i

If postqueue -f doesn't result in deliveries:

1. Check mail logs: tail -f /var/log/mail.log
2. Verify Postfix is running: sudo postfix status
3. Test connectivity to destination: telnet target.server 25

For regular monitoring, consider this bash script:

#!/bin/bash
QUEUE=$(postqueue -p | grep -c '^[A-Z0-9]')
if [ "$QUEUE" -gt 10 ]; then
  logger "High mail queue detected ($QUEUE messages), forcing flush"
  postqueue -f
fi