Incoming emails suddenly stopped with no warning

I’ve been using K9 mail for a few months now after my previous mail stopped working on my phone suddenly. All was ok until a couple of days ago when K9 suddenly stopped receiving new messages. No error messages, just no new emails. Took me a while to notice. Can still send messages, but wont receive anything. I did a test of the incoming server, which then brought up an error message, The server itself is not an issue as it still works perfectly on all other devices, and as stated above will still allow me to send messages, just not receive them to the phone. I have a Samsung phone running Android 13. K9 is up to date with version 6.804 installed. My email comes via a private server running Squirrelmail. I havent done anything different with the phone or any apps which should cause it to suddenly stop receiving emails to my phone via K9 when it was working perfectly for months. Any suggestions?