Thunderbird for Android
 

When I ask for Thunderbird for Android most of you will answer K9 mail, yes it is a very good email app but I want something else. The reason sounds silly, K9 lacks a basic feature that it does not clean the trash on exit, I don't understand why some people don't want that feature. I got lot of mail everyday to clean up, they are not junks but useful info. that I need to see and will not keep them after read, so that I need to delete them twice everyday ! any comment ?

I think there is a clean trash feature in K9. Otherwise on android we usually don't exit apps, just close the window, therefore cleaning on exit has no any meaning. Probably.
Also there is a way to delete immediately, without trash, but in that case you will not be able to restore any message (set "trash folder" to none).

I used other mail client before, as far as I remember they will clean the trash on next mail check, that's how TB do it. Some mail app did exit via double "back" btw.