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/main.cf and add the remote mail server hostname to end of the file:

relayhost = [remote.server.com]

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

relayhost = [remote.server.com]: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 = [remote1.server.com]
fallback_relay = [remote2.server.com]

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,

192.168.6.45/32
192.168.7.34/32

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.

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