Postmark extends Fuel's email class via a driver and allows you to send emails via the transactional email service. (http://postmarkapp.com)
to install this package simply add this source to your package configuration file:
http://github.com/tomschlick
and then use this command:
php oil package install postmark
You can use this exactly the same way as you would use the default email protocols, just set your postmark api key in /fuel/packages/postmark/config/postmark.php
Email::factory('default', array('driver' => 'postmark'))
->to('to@yoursite.com')
->from('from@yoursite.com')
->subject('testing123')
->html_body('Your message goes here.')
->send();