Hello!
I am on the latest beta version under Android 6.
I remember that in older versions, when you would set max messages number for an account, that would be the maximum number of messages kept in the folder, with the exceeding ones automatically deleted (and in fact you would use the star icon to keep them whatever would happen).
Starting in a moment I cannot identify, this is not happening any more, and folders now grow indefinitely. I discovered that once when situation was very compromised (I was not checking this aspect, since I trusted K9 would automatically delete the excess), since the folder had already 4000+ messages, and deleting messages in big batches would bomb the app.
Problem solved by deleting and reinserting from settings the account. That did not take much time.
But this self limiting feature was very convenient for me in the mobile environment (I am not doing so on my desktop client, of course).
Am I doing something wrong (help really appreciated! 
) or has this option been removed for some reason?
Thank you in advance!
Sleepy