AquaMail

I've been a long-time user of K-9 mail on Android, but the combination of the dreaded doze ("syncing disabled") bug and various bits of clunky UI persuaded me it was time to move on. AquaMail, on first impressions, is rather nice. It lets me set the Archive folder, and can Archive things in a couple of taps. It also has a better approach to folder management, not showing the entire list in most places, but rather the folders you've actually used. Well worth a few quid for the pro version.