Sending an outgoing email through a 3rd party SMTP relay service is a quick and easy alternative to setting up a full-fledged local email server.

Open /etc/postfix/ and add the remote mail server hostname to end of the file:

relayhost = []

If the remote system uses a different port number, please specify the port number:

relayhost = []:587

In some situations, for redundancy, you may want to configure the relay via two or more remote SMTP server. In such instances, you may configure the following;

relayhost = []
fallback_relay = []

This gives us redundancy in a way so that if one of the SMTP relays is down, we can send emails out via the second one

Now, open /etc/postfix/mynetworks and the public IP of the remote mail server. For example,

This will let the mail server know exactly which networks are allowed to relay mail. This is a very important step

Restart postfix:

service restart postfix

* Don't forget that remote mail servers need to have a valid SPF record and to be properly configured.