This is the server side error. Your IMAP server is informing you, that it is temporary unavailable. All ImapResponseExceptions indicate error returned by the server.
If it happens often, you should contact your IMAP server administrator.
The common cause is error, is that you're performing large amount of operations in a short time, and your server is throttling you. Consider adding a Thread.Sleep and check if the server is more likely to cooperate.