GD Mail Queue Pro plugin has support for custom SMTP services for PHPMailer. One of the services included is for the PostMark email sending service. And, to start using this service to send emails using SMTP you need PostMark account and make sure that you SMTP option activated. After you have set up your PostMark account, you need to get SMTP access information to add to the GD Mail Queue Pro plugin PostMark Service settings panel.
To get the required information, open your MailJet account, and open SMTP Settings. You must enable SMTP, and once you do that, you will find username and password for it, to use in the GD Mail Queue Pro plugin settings for PostMark.