Mailster v1.6 is released to allow compatibility wih PHP 8.
It also is our first release that is no longer compatible with Joomla 2.5 (which we supported from 2012 to 2020).
There are a lot of changes under the hood to start to make Mailster ready for the coming Joomla 4 version. However Joomla 4 is not yet supported, since Joomla 4 is still in beta.
All other changes are listed in the changelog.