Happened to me a week or two ago. The IMAP server was unavailable for about four hours straight.
Support response was that "the server is experiencing high load" and that it'd eventually become available again. Which of course it did, after four hours. I suggested adding capacity to the server, but that part was politely shrugged off
Just to clarify: during this four-hour blackout the webmail didn't work either because it couldn't connect to the IMAP server.