This document describes 5 easy steps to setup your Postfix email server as a smarthost to relay on a given mail relay server for all outgoing mail.
1. Create a password maps file (it assigns username/passwords to specified mail servers). You can choose any name, let’s say it is /etc/postfix/relay_passwd. It’s content should be as follows:
Note: Replace USERNAME and PASSWORD with your mail relay username and PASSWORD.
2. Set proper permissions for that file:
# chown root:root /etc/postfix/relay_passwd
# chmod 600 /etc/postfix/relay_passwd
3. Create hash from maps file (remember to do it each time you change your maps file):
# postmap /etc/postfix/relay_passwd
4. Configure your /etc/postfix/main.cf:
relayhost = [relay.dnsexit.com]
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/relay_passwd
Note: If your ISP blocks outgoing port 25. You can choose to use alternative SMTP ports by appending the port at the end:
relayhost = [relay.mail.com]:26
5. Reload or restart your postfix:
# /etc/init.d/postfix restart