As I have announced last week, GD Mail Queue plugin now has a Pro version, bringing many more new and Pro exclusive features, with a lot more new things on the future releases roadmap.
GD Mail Queue Pro version numbers will start from version 4.0 to avoid the conflicts with Free/Lite version current numbers, and it is expected that Pro development will generate at least 3 or 4 major versions until the end of this year.

The Pro version brings support for popular third-party email services in the form of the SMTP services under PHPMailer or full engines for REST API. SMTP services implement SMTP relay connection to the third party servers to deliver emails, and in some cases, have extra settings to pass with each email.
The plugin is made to work on it’s own, it doesn’t need other email related plugins.
If you need support for REST API, the plugin has few already, and more are coming. The same goes for SMTP support. And the plugin includes everything else other email plugins might implement: queue, HTML emails with templates, log and much more.
As for the REST API, the plugin has integrated SendGrid REST API only, but it also supports addons. Addons will be used to add more REST API services because REST API SDK’s for some services can be really huge (Amazon is 12MB). So, the plugin already has addons for Mailgun REST API and Amazon AWS SES REST API. More are coming soon. All addons will be free to download and use with GD Mail Queue Pro license.

Plugin home page is updated with all the Pro features and additional information, and there are few useful articles added with a list of features compared between Free vs. Pro versions, comparison of SMPT vs. REST API and emails type detection.
Check out the plugin features, and let me know what else would you like to see in the plugin and which third party services to support. If you find any issue with the plugin, as usual, please report them in the support forums.