https://www.betterbird.eu/index.html

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.