I am getting a
Cannot connect to server (command capability response: #2 [NO, [LIMIT], CAPABILITY Rate limit hit.])
error when fetching mail from my yahoo account.
Does anyone know what it means and how I can get rid of it?
I haven’t changed any settings recently, so I am not sure what triggered it.
It means that K9 is opening too many IMAP connections (more than your mail server allows). Not sure, how you can influence that.
What K-9 version are you using? (don’t say “latest”, look up the version number on the about screen)
@capi Is there a limit to the number of email accounts one can enter in K-9? I have quite a few (14). Don’t even think about asking me why I need that many. 
I probably couldn’t explain it.
@ByteHamster I’m using K-9 version 5.600.
Updating it to the latest version lead to my smartphone freezing 
and rebooting several times, so I’ve given up on that for now. I know the workarounds, I just don’t fancy doing that at this time. Matter of priorities.
In the meantime, the issue seems to have resolved itself, the messages that were stuck on the server were fetched with delay. Not sure if it’ll reoccur again, though.
That’s not about a limit in K-9 but in how many concurrent connections the server allows (e.g. from the same IP address, for the same user, whatever other criteria the server seams fit).
If you are connecting over wifi, that is likely presenting only one public IP, other devices on the same network that are connecting to yahoo will be part of the count in whatever rate/connection limiting they have in place. This would include other wifi-connected mobile devices as well as desk/laptops connected to the lan. Note, some desktop clients will hold open a connection to the server for each folder you have open, which quickly pushes counts up.