I know, it was the same for me - home.pl changed something in their configuration. I don’t hold much hope that they will respond to my message, if they get a number of complaints, they may eventually fix it. It’s definitely not just a Mailspring issue, I had the same problem with Geary and their dev actually managed to prepare a working branch for me today, they will probably include a fix in a future release - but as I mentioned, it is just a workaround allowing the client to ignore a faulty response from server.
(Originally posted by ldziubek on GitHub.)