GD Mail Queue Pro plugin has support for custom SMTP services for PHPMailer. One of the services included is for the MailJet email sending service. And, to start using this service to send emails using SMTP you need MailJet account and make sure that you have Transactional emails option activated. After you have set up your MailJet account, you need to get SMTP access information to add to the GD Mail Queue Pro plugin MailJet Service settings panel.
To get the required information, open your MailJet account, click on Transactional tab on top of the page, and open SMTP Settings, and there you will see login (username) and password.