The Postmark module allows the administrator to switch the standard SMTP library over to use the third party Postmark API to send out emails from their site. An account with Postmark is required to use this module.
IMPORTANT NOTICE FOR DRUPAL 7 USERS
On April 5, 2013, version 0.5 of the Postmark PHP library was released. Currently only the 7.x-1.x-dev version of Postmark supports the 0.5 library. If you download the 7.x-1.2 release of Postmark, use the 0.4.5 library available here:
The upcoming 7.x-1.3 release of Postmark will include support for the 0.5 library.
Users of the Drupal 6 version of the Postmark module can ignore this notice since the 0.4.5 release of Postmark PHP is bundled into the Drupal 6 module folder.
If you have any requests or problems using the module, please submit an issue in the Postmark queue (http://drupal.org/project/issues/postmark).
Sender Signature Requirement
The Postmark service requires that the "from" email address be registered with them as a Sender Signature. This will normally be your Site Information email address. More information is available here.