a Quick post on how to change your relayhost on Postfix to a External SMTP Provider and aswell how to revert back the changes so the Relay server sends out mail directly.

Checking your current relayhost configuration:

We will assume your /etc/postfix/main.cf has a relayhost entry of #relayhost =, in my example it will look like this:

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$ cat /etc/postfix/main.cf
#relayhost =

If not, you can just adjust your sed command accordingly.

Changing your relayhost configuration to a External SMTP Provider:

We will use sed to change the relayhost to za-smtp-outbound-1.mimecast.co.za for example:

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$ sed -i 's/#relayhost\ =/relayhost\ =\ \[za-smtp-outbound-1.mimecast.co.za\]/g' /etc/postfix/main.cf

to verify that we have set the config, look at the config:

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$ cat /etc/postfix/main.cf | grep relayhost
relayhost = [za-smtp-outbound-1.mimecast.co.za]

Once you see the changes looks as expected, you can restart postfix:

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$ /etc/init.d/postfix restart

Then you can tail the logs to see if the mail gets delivered:

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$ tail -f /var/log/maillog

Revert your changes so that postfix sends out directly:

To revert your changes, let’s change the config back to what it was:

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$ sed -i 's/relayhost\ =\ \[za-smtp-outbound-1.mimecast.co.za\]/#relayhost\ =/g' /etc/postfix/main.cf

To verify your changes:

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$ cat /etc/postfix/main.cf | grep relayhost
#relayhost =

As you can see the relayhost is commented out, meaning that the relayhost property will not be active, go ahead and restart the service for the changes to take effect:

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$ /etc/init.d/postfix restart

Same as before, look at the logs to confirm mailflow is as expected:

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$ tail -f /var/log/maillog