MockSMTP is a native Mac application that embeds its own SMTP server. It also features an e-mail client browser, enabling instant viewing of both raw content and HTML rendering, so you can see how your mail looks when delivered.
Configuration couldn't be simpler: just run the application and everything's set up. You don't even need an internet connection for it to work!
You can download a 30-day free trial and now you can buy it on the app-store.