This is the first public release of MailMate with a so-called “hardened runtime”. It is also the first release “notarized” by Apple. In short, this means that this and every future release of MailMate has been scanned by Apple before being released (this includes beta/test releases). Apple documentation currently states they they scan for malicious content and code-signing issues. If you would like to know more about this then you can take a look at the developer documentation.
Once again, the minor version bump “hides” the fact that this release includes a lot of improvements to MailMate. All of the new features, changes, and fixes are listed in the release notes.
Note that the next public release of MailMate will most likely not support macOS 10.8 and 10.9, but this should affect very few users (less than 0.5%).