I ran into a major issue after upgrading to Mavericks yesterday where Apple Mail refused to connect to an IMAP SSL account that has been working for years. I tried everything I could think of and nothing worked. Finally I found a solution so I thought I'd share with the community. 1. Disable the IMAP account by unchecking the "enable this account" option 2. Create a brand new IMAP account using the exact same parameters/info (this apparently overwrites the existing account as no new account showed up 3. Enable the IMAP account by checking the "enable this account" option Now it works and connects/auths just fine. Very strange, but hopefully this will help others get back and running.