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I’m not sure what is causing your problem. I looked around a little to see if I could find anyone experiencing similar issues. The best I could do is find a discussion on technet where someone else was having problems getting Mail App to connect to hosted email via IMAP. You already addressed the server setting details, which would have been my guess, but it might be worth going over them again just to make sure they match. Sorry not to have a better comprehensive answer for you, but perhaps this can set you on the right track. Good luck!