And its even funnier that cyrus-imapd (which is what fastmail.fm runs, and contributes heavily too) is what I think (please correct me if wrong) is what is ship

And its even funnier that cyrus-imapd (which is what fastmail.fm runs, and contributes heavily too) is what I think (please correct me if wrong) is what is shipped with OSX Server. So the one IMAP server that apple likely uses for their own things, was potentially well tested, and its the one company thats running it has called them out, and now has to bite their words. I use/run cyrus-imapd for years and have never had an issue with mail.app. But it is pretty frequent that I hear people with other imap providers (especially gmail) that have issues with it.