I'm actually not sure about this, Thunderbird to me seems like it's pretty good: https://www.thunderbird.net/en-US/
The only bloat that i can think of is the calendar functionality that i simply don't care about, but that's easy enough to get out of the way. Maybe one could also have gripes with the account administration section, though that's not necessarily touched in the daily usage.
Apart from that, it does everything i need, in the simplest way possible and is as boring as it is usable. Contrasted with lots of the modern day software, i'd instead put it into the category of "workhorse" software, where LibreOffice, GIMP, KeePass, Notepad++, SourceTree/Git Cola and others live - dependable pieces of software that do what i need without many breaking changes and all that many bells and whistles. The kind that you install and forget about, maybe updating them once every few years if at all needed.