There is currently a problem with one of our IMAP/POP servers and customers may be affected by this. The problem could show as either a failure to connect to the server, or your email program continually asking you to enter your username and password and claiming they are wrong.
We are looking in to this issue and will update this post further as we know more. Apologies for the inconvenience.
UPDATE 22:20 CET: This issue is now resolved. We are taking steps to prevent this issue from occurring again in the future.