GD Mail Queue Pro plugin has support for custom SMTP services for PHPMailer. One of the services included is for the PepiPost email sending service. And, to start using this service to send emails using SMTP you need the PepiPost account. After you have set up your PepiPost account, you need to get SMTP access information to add to the GD Mail Queue Pro plugin PepiPost Service settings panel.
To get the required information you need to either use Sandbox on PepiPost (it can send emails to authorized emails for testing purposes), or you need to verify your sending domain. Once you do that, you will get to the panel with the SMTP settings where you can find the username and password needed.