I mean there is Mozilla Thunderbird, but it hasn't been the focus of much development since 2012, with intention to decouple it from Firefox development in 2015

I mean there is Mozilla Thunderbird, but it hasn't been the focus of much development since 2012, with intention to decouple it from Firefox development in 2015. [https://en.wikipedia.org/wiki/Mozilla_Thunderbird https://en.wikipedia.org/wiki/Mozilla_Thunderbird] [https://www.mozilla.org/en-US/thunderbird/ https://www.mozilla.org/en-US/thunderbird/]