GD Mail Queue Pro plugin has support for custom SMTP services for PHPMailer. One of the services included is for the SendInBlue email sending service. And, to start using this service to send emails using SMTP you need SendInBlue account. After you have set up your SendInBlue account, you need to get SMTP access information to add to the GD Mail Queue Pro plugin SendInBlue Service settings panel.
To get the required information, open your SendInBlue account, click on Transactional tab on top of the page, open Settings, then open Configuration, and under SMTP Relay, click a link to get your SMTP key.
For GD Mail Queue Pro settings for SendInBlue you need Login email and Master Password key.