As with many other people posting in this forum, IMAP sync was working fine for me and then all of a sudden broke after Jan. 20.  I've recreated the profile and done many other experiments.  Always get error 800C0000.

Please don't postpone troubleshooting by asking for a screenshot of my settings.  By now you should realize that something happened on Jan 21 that broke syncing for many people.  Some other posts have been talking about SSL vs TLS, for example.  (WP 7 can't choose between the two, nor should all your customers have to change a setting that used to work.)