Hello, I have been using K9 Mail for long and with a great success. Typical use case was Thunderbird on a PC and K9 on Android just to view/screen messages. This works very well.
As the time goes by I have been struggling recently with another use case. Email account kept on Android only - tablet with a keyboard instead of a PC. There is nothing to delete old email in this case. After some time the mailbox will be full irrespective of its size. What I have managed to make up is to use POP3 as protocol and ask the users to manually mark all messages and put to trash by tapping the icon. When its done, all messages are removed from server, as it sees the action as deletion from the Inbox folder. The user have still access to old emails, but in the trash folder.
I shared this with the objective o maybe purging the inbox with K9 can be done differently, but I do not know how. Any ideas? I would like to use IMAP as it is newer than POP3, but could see a way to achieve what I was looking for. Apparently moving an email to trash moves on the server to the folder Trash. 
It would be great if I could set a number of days (like 90 or 180) days in K9 settings and messages older than this would be moved to trash automagically. What do you think. All in all, keep up the good work!