To me there is not a single client which isn't off somehow, missing something. I feel like nobody really loves mail client development, because email kinda sucks. Mail just needs to die IMO. You wouldn't design something like email today. I wouldn't want to support all the legacy "features" either. It's a wonder someone is caring about these clients at all.

It's not like mail is going anywhere soon. Look at the fax. There is many decades of infuriating mail client dependecy ahead.