Postfix Mail Server Sending Email to Remote Domains.

Hey, I have been setting up Nagios and need it to be able to send alerts. I have installed postfix and am able to see that it is trying to send these alerts. However it says it can’t send to remote domains. I am wondering how to allow it to send emails to remote domains. (I only have postfix installed in case I need anything else). I am trying to use webmin as much as possible so any options in that you know of to do this would be helpful though command line ok.

Any help would be appreciated. Thanks