If you want to test it from the outside, you can use no-ip.com's DNS service, to assign your host a name.no-ip.com that can be used as a MX (Mail Exchanger). My suggestion isn't quite as safe as Mara's, but it will be an accurate test of whether your host can receive mail from the outside world.
Before connecting a new mailserver to the internet, ensure that you are running the latest postfix release... Also, be sure to test yourself for open relay using
www.ordb.org.