With the new WordPress 5.5 on its way, bringing a huge change in how emails are processed due to the update of PHPMailer library, GD Mail Queue Pro and Free had to be updated to support new WordPress.
Version 5.1 Pro and 3.7 Free include support for both the older PHPMailer library in WordPress before 5.5, and the new PHPMailer library for WordPress 5.5. This is achieved using class_alias functions, and by cleaning up and removing some obsolete properties used from the old PHPMailer library.
Due to the nature of this change, please, make sure to report any issues with the new version running on WordPress 5.5 with a new PHPMailer library or older versions of WordPress and old PHPMailer library.
This release includes various updates and fixes, including some issues with the database related methods, some PHP strict warnings and problems with normalization of email used for processing.
To see the list of all the changes in this version, please check out the changelog. If you find any issues with the new version, please, report them in the support forums.
