Version 1.13 of MailMate adds support for macOS 10.15 (Catalina) and removes support for macOS 10.8 and 10.9. In other words, MailMate now requires macOS 10.10. It is unknown exactly when Apple is going to release macOS Catalina, but it’s likely to be fairly soon.
There are no major new features in this release of MailMate, but there has been some important major internal changes to MailMate in preparation for some future changes/features.
A detailed list of the changes and fixes since the latest public release can be found in the release notes.
If you are new to MailMate then read more about it on the about page. The release of Catalina could be a suitable occasion to try a different kind of email client.