GD Mail Queue Pro plugin has support for custom SMTP services for PHPMailer. One of the services included is for the Amazon AWS Simple Email Service (SES) email sending service. And, to start using this service to send emails using SMTP you need Amazon AWS account and you need to enable the SES service for it. After you have done that, you need to generate username and password for SES service SMTP.
To do so, login to your AWS account, open SES panel and then find the SMTP Settings under Email Sending. There, you will see the button to generate SMTP credentials. Make sure to download the credentials file when it is generated, because you will not be able to get them after that anymore, and if you lose it, you need to generate credentials again.
By default, when you activate SES services, your account is in the sandbox mode, and you can send emails only to verified email addresses or domains. For sending emails, you must verify email to use in email From field, and that must be set in the GD Mail Queue plugin From settings. If you attempt to use email in From that is not verified, you will get an error.