Do you have a DSL or cable provider that blocks port 25 traffic?
Do you have another machine that all you mail will be delivered to (if you could get to it)?
Here is what you need in your .mc file to setup a smart host to send all you mail to another server via a different port:
define(`RELAY_MAILER_ARGS', `TCP $h 587')
Since all my mail was going to the <hostname> machine anyways I didn't care about relaying. If you need to relay mail you will need to allow relaying from that ip or if it is dynamic you should setup authentication. For more information can be found here: