My K-9 Android client hasn’t been enforcing the size limit on number of messages in a folder for a long time. I had it set to 250, then tried changing it to 500 just now, then restarting the app, but no luck. I can’t count how many are in there, but they go back to December 2021, so it’s probably several thousand. As a result, it takes a long time to “mark all as read” every day or so, as well as slowing searches. I probably tried removing and re-installing, but that was probably a while ago.
The limit does not apply to already present messages.
If you want to remove old messages, compress the account from the settings. It will then apply all limits, etc. to the account as is. Warning: This may take a while. Keep K-9 open and your screen on and unlocked until you receive a notification that the account’s size has been shrunk to X (where X is the size in Bytes, KB, … at the end).
Alternative: If it’s an IMAP account, you can remove the account and set it up anew. Make sure to have set the limits before fetching messages for the first time. — In some cases this is much faster than the first method (depending on the limit set and the amount of messages).
You may also want to set a maximum age of messages to fetch. Is no as hard of a limit in a fixed number, but more closer to how we humans actually think…