Re: I should be getting "less mobile radio active" now I guess,
Maybe. Maybe not.
I'm not sure there is a direct connection.
Re: my question is what happens with non-synced items/folders etc?
The app trims its on-device storage over time, automatically.
Re: Some days ago I figured out that I couldn't download/view emails from a non-syncing folder,
These folders get trimmed over time. You can always refresh them yourself after opening.
Re: why can't I remove syncing in exchange accounts for all folders except inbox?
Exchange has "server defined" Sent / Deleted / Drafts they're always there in any Exchange accounts, so we just find those and set them "to sync".
Re: aquamail is the top drainer in my battery usage with already 1+ hour of mobile radio active
1 hour mobile radio - and "keep awake"? much smaller?
The "keep awake" is how long the app actually worked (total), including using the mobile radio.
The "mobile radio" is how long Android decided to keep the radio in a high power state, "giving credit" to Aqua Mail for it.
For others the "offending app" may be something else - Chrome, Facebook, Whatsapp, Play Services, etc.
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The bug report now has over 4000 "stars" and comments are closed. The bug is still listed as "assigned" (as opposed to "released" or "fixed, release pending"):
https://issuetracker.google.com/issues/37038331