When your Postfix server attempts to deliver email to certain domains, you might encounter this rejection:

504 5.5.2 <contact@localhost>: Sender address rejected: need fully-qualified address

This occurs because some strict mail servers require sender addresses to use proper FQDNs (Fully Qualified Domain Names) rather than localhost or unqualified hostnames.

Here are the essential Postfix configuration changes you need to make in /etc/postfix/main.cf:

# Set your actual mail domain
myorigin = yourdomain.com

# Force FQDN in sender addresses
sender_canonical_maps = regexp:/etc/postfix/sender_canonical

# Default transport maps (if not already set)
transport_maps = hash:/etc/postfix/transport

Create /etc/postfix/sender_canonical with this content:

/^.*@localhost$/ contact@yourdomain.com
/^.*@hostname$/ contact@yourdomain.com

Then compile the map:

sudo postmap /etc/postfix/sender_canonical

These parameters help ensure proper FQDN usage:

# Set your mail server's FQDN
myhostname = mail.yourdomain.com

# Important for HELO/EHLO
smtp_helo_name = mail.yourdomain.com

# Prevent localhost in headers
masquerade_domains = yourdomain.com

After making changes, always test:

sudo postfix check
sudo postfix reload

You can verify the sender address behavior with:

echo "Test" | mail -s "Test Subject" external@example.com

• Check /var/log/mail.log for any remaining issues
• Use postconf -n to verify your active settings
• Test with strict mail servers like Gmail or Outlook

When Postfix attempts to send emails to certain domains, you might encounter the rejection error indicating the sender address isn't fully-qualified. This happens because remote SMTP servers enforce RFC compliance, requiring sender addresses in the format user@domain.tld rather than user@localhost.

The root cause typically lies in these Postfix settings:

# Main configuration file: /etc/postfix/main.cf
myhostname = mail.yourdomain.com
mydomain = yourdomain.com
myorigin = $mydomain

When these aren't properly configured, Postfix defaults to using localhost as the domain part.

Here's how to properly configure Postfix for FQDN compliance:

# Edit main.cf
sudo nano /etc/postfix/main.cf

# Set these parameters:
myhostname = mail.yourdomain.com
mydomain = yourdomain.com
myorigin = $mydomain
inet_interfaces = all

For complex setups with multiple domains, use sender canonical maps:

# /etc/postfix/sender_canonical_maps
@localhost    contact@yourdomain.com
@hostname     contact@yourdomain.com

# Add to main.cf:
sender_canonical_maps = hash:/etc/postfix/sender_canonical_maps

# Then compile the map:
sudo postmap /etc/postfix/sender_canonical_maps

After making changes, verify with:

sudo postfix check
sudo postfix reload
echo "Test email" | mail -s "Test" recipient@example.com

Check your mail logs to confirm the sender address appears correctly:

tail -f /var/log/mail.log

• Ensure your DNS has valid MX records
• Verify reverse DNS matches your hostname
• Check for conflicting settings in master.cf