When configuring Postfix to forward emails from local users like root to external addresses, you might encounter a situation where aliases defined in /etc/aliases seem to be ignored. The key indicators are:

status=bounced (host internal.my.domain[XXX.182.189.136] said: 
550 5.1.1 : Recipient address rejected: 
User unknown in virtual mailbox table)

First, verify these critical configuration elements in your main.cf:

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
mydestination = $myhostname, localhost.$mydomain, localhost

The most common reason for aliases being ignored is that alias_maps isn't properly set. While you have:

alias_database = hash:/etc/aliases

You also need:

alias_maps = hash:/etc/aliases

1. Edit your /etc/postfix/main.cf:

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
mydestination = $myhostname, localhost.$mydomain, localhost

2. Edit your /etc/aliases:

# Basic system accounts
root: hostmaster@my.domain
munin: root

3. Run these commands:

sudo newaliases
sudo postfix reload

To verify your setup is working:

echo "Test email" | mail -s "Test Subject" root

Check your mail logs:

tail -f /var/log/mail.log

If you're still having issues:

• Check Postfix's virtual alias maps
• Verify DNS resolution for your domain
• Ensure your mail server can actually send to the destination

postconf -n | grep alias
postconf -n | grep mydestination

---

When configuring a Postfix MTA with Munin monitoring, many admins encounter a puzzling issue where email aliases defined in /etc/aliases fail to work as expected. The specific case shows:

# /etc/aliases
root:    hostmaster@my.domain

Yet the mail logs reveal alias expansion isn't occurring:

Dec 22 16:45:20 postfix/smtp[22099]: 5CFBA2011E0: to=<root@my.domain>, 
    orig_to=<root>, relay=internal.my.domain[XXX.182.189.136]:25, 
    status=bounced (550 5.1.1 User unknown)

The critical Postfix settings involved:

# main.cf
alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
mydomain = my.domain

Three common reasons cause this behavior:

1. Missing alias_maps directive: While alias_database exists, Postfix actually uses alias_maps for lookups
2. Incomplete hash generation: Running newaliases may not always update all required files
3. Local delivery misconfiguration: The MTA might be treating root@ as a virtual address

First, verify and correct the configuration:

# Ensure both parameters exist in main.cf
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases

# For virtual alias testing, add:
recipient_canonical_maps = hash:/etc/postfix/recipient_canonical

Then implement this diagnostic script to test alias resolution:

#!/bin/bash
# Test alias expansion
echo "Testing direct alias lookup:"
postmap -q root hash:/etc/aliases

echo -e "\nTesting delivery path:"
sendmail -v root <<EOF
Subject: Alias Test
Test message for alias verification
EOF

If standard aliases still fail, consider these approaches:

Virtual alias mapping:

# /etc/postfix/virtual
root@my.domain hostmaster@my.domain

Recipient canonical mapping:

# /etc/postfix/recipient_canonical
root hostmaster@my.domain

Remember to compile maps after changes:

postmap /etc/postfix/virtual
postmap /etc/postfix/recipient_canonical
systemctl reload postfix

Use these Postfix commands to verify configuration:

postconf alias_maps
postconf alias_database
postmap -q root hash:/etc/aliases

Check the mail queue processing path:

postqueue -p
postsuper -v ALL