=== Using a relay host

These are the basic steps to set up Postfix to use SMTP Authentication to send
mail through a relay host.

Set up a password maps file (`/etc/postfix/sasl_passwd`) as follows:

------
mail.ispserver.com    username:password
------

------
# chown root:root /etc/postfix/sasl_passwd
# chmod 600 /etc/postfix/sasl_passwd
# postmap /etc/postfix/sasl_passwd
------

Append the following lines to `/etc/postfix/main.cf`:

------
relayhost = mail.ispserver.com
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =
------

Finally reload postfix:

------
# postfix reload
------

That should do it!