This is pretty easy to use (I'm assuming you know how to use node.js - if not read up at nodejs.org first) - simply sign up over at http://postmarkapp.com/ , follow their instructions to get your api key, then check out the demo.js script, putting your own api key and sending email either in the node-postmark.js file in the defaults object, or you can override when you call the send method by specifying the "token" and "from" properties of the first argument.

Basically, check out the demo - should be self explanatory.

Enjoy :)