Betterbird is a fine-tuned version of Mozilla Thunderbird, Thunderbird on steroids, if you will.
Betterbird is better than Thunderbird in three ways: It contains new features exclusive to Betterbird, it contains bug fixes exclusive to Betterbird and it contains fixes that Thunderbird may ship at a later stage. Please refer to this feature table for examples. This should give you an impression of where the project is headed.
It has a number of usability improvements over regular Thunderbird. In the context of this project, I believe this one is EU based.
For clients, you could include BetterBird
https://www.betterbird.eu/
https://blog.betterbird.eu/
It has a number of usability improvements over regular Thunderbird. In the context of this project, I believe this one is EU based.
Will make sure to add it to the website - definitely an option to consider ahead of Thunderbird.