Thank you for the quick answer.
As already described in the article above ( 'compress account' not working ) - despite complete deletion of the mails - the respective account gets bigger and bigger over time - “blown up”, so to speak. Therefore there is in the version 5.600 the possibility to COMPRESS the desired account together with its folders: → Account → Folder → 3dot Menu (the three points right down) → Compress account.
I can’t find this function in the latest version of K9 so far.
Where can I find this function? Or does it not exist anymore?
In the corresponding mail program “Thunderbird” for Windows and Linux there is also such a function: → File → Account → Compact Folders