When managing multiple domains on a single mail server, you often need different outgoing mail configurations per domain. The default Postfix setup typically routes all outgoing mail through a single SMTP relay, which becomes problematic when:

• Different domains require separate SendGrid accounts
• You need domain-specific authentication credentials
• Mail tracking metrics need to be isolated per domain

The solution lies in Postfix's sender-dependent relayhost feature. Here's the key configuration:

# /etc/postfix/main.cf
sender_dependent_relayhost_maps = hash:/etc/postfix/relay_hosts
smtp_sender_dependent_authentication = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

Create /etc/postfix/relay_hosts with domain-specific configurations:

# Format: sender_address relayhost:port
@domain1.com [smtp.sendgrid.net]:587
@domain2.com [smtp.sendgrid.net]:587

Compile the map file:

postmap /etc/postfix/relay_hosts

Create /etc/postfix/sasl_passwd with domain-specific credentials:

# Format: relayhost:port username:password
[smtp.sendgrid.net]:587 domain1_apikey:SG.xxxxxxxx
[smtp.sendgrid.net]:587 domain2_apikey:SG.yyyyyyyy

Secure and compile the file:

chmod 600 /etc/postfix/sasl_passwd
postmap /etc/postfix/sasl_passwd

Verify your setup with:

postconf -n | grep sender_dependent
postmap -q @domain1.com hash:/etc/postfix/relay_hosts

• Check mail logs: tail -f /var/log/mail.log
• Test SMTP delivery: swaks -t recipient@example.com -f sender@domain1.com
• Verify SASL authentication: postmap -q '[smtp.sendgrid.net]:587' /etc/postfix/sasl_passwd

When managing multiple websites on a single server, a common challenge arises with email delivery. Many developers use SendGrid as their SMTP relay, but need different authentication credentials per domain. The default Postfix configuration typically routes all outgoing mail through a single SendGrid account, which isn't ideal when managing separate client domains.

The solution lies in Postfix's sender-dependent SASL authentication feature. This allows you to specify different SMTP credentials based on the sender's email domain. Here's how to implement it:

# Main configuration in /etc/postfix/main.cf
smtp_sender_dependent_authentication = yes
sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relayhosts
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwords
smtp_sasl_security_options = noanonymous

First, create the sender_relayhosts file to map domains to their respective SendGrid endpoints:

# /etc/postfix/sender_relayhosts
@domain1.com [smtp.sendgrid.net]:587
@domain2.com [smtp.sendgrid.net]:587

Then create the sasl_passwords file with domain-specific credentials:

# /etc/postfix/sasl_passwords
[smtp.sendgrid.net]:587 username1:password1
[smtp.sendgrid.net]:587 username2:password2

Always protect your credential files and generate the hash maps:

sudo chmod 600 /etc/postfix/sender_relayhosts /etc/postfix/sasl_passwords
sudo postmap /etc/postfix/sender_relayhosts
sudo postmap /etc/postfix/sasl_passwords

Verify your configuration with these commands:

postconf -n | grep sender_dependent
postmap -q @domain1.com hash:/etc/postfix/sender_relayhosts

If emails aren't routing correctly:

1. Check mail.log for authentication errors
2. Verify mapping file permissions (should be 600)
3. Test SMTP authentication manually using telnet
4. Remember to restart Postfix after configuration changes

For more complex scenarios, consider using transport maps:

# /etc/postfix/transport
domain1.com smtp:[smtp.sendgrid.net]:587
domain2.com smtp:[smtp.sendgrid.net]:587

# In main.cf
transport_maps = hash:/etc/postfix/transport

This method provides even more control over domain-specific routing.