I use K-9 with POP as described in : Why was the account overview screen useful? - #13 by pmontra . My android devices are only used for mail when away from my primary PC. All outgoing messages are bcc’d to the relevant email address so my primary PC ends up with copies of all incoming & outgoing emails. This means that messages build up & aren’t cleared in K-9. Once I’ve returned home & downloaded with my primary PC, which is set to delete from the server, I wipe the copies on my android devices. This is where the “Clear messages” is useful.
I have noticed that occasionally the size of a mailbox doesn’t return to the minimum (104.4 KB)with “Clear Messages”. In this case the “Recreate Data” option is required to wipe the remaining unwanted data. I haven’t investigated why this happens other than checking for any messages remaining (never found any). It doesn’t affect all accounts at any one time, or for any obvious reason. I wouldn’t have been aware of this memory ‘leak’ if K-9 didn’t display the account size or if I didn’t enable its display, but now knowing that it can happen would leave me wondering if it still happens with v5.8, but not being able to track, other than looking at the storage usage in android App Info.
I agree, not an optimum use of the back key. I suggested it as it follows the workflow of previous versions of K-9: In v5.6 pressing back from an inbox takes you to the account overview page, pressing the K-9 logo from the inbox of an account displays the account folders. The latter is the same pressing as the hamburger from the inbox in v5.8.