Postbox was initially based on Mozilla Thunderbird… 
By the way, you’re right. The latest version is stellar in nearly every single way.
https://blog.thunderbird.net/2023/07/our-fastest-most-beautiful-release-ever-thunderbird-115-supernova-is-here/
Just look at how nice the user interface is!
By the way, I find it funny that you created this topic shortly after I created mine. Would you ever use an iOS version of Mozilla Thunderbird?