I am having he same problem as well, this specifically started for me and all my Mac users, 2 days ago. I have placed a ticket and got connected with a Microsoft tech that then forwarded me around 3 times with no resolution. I have been trying things on my own, such as trying to connect the account manually using SMTP or IMAP, none have worked so far. I have also attempted to point directly to one of MS Auto-discover IP's in my local hosts file with no luck. I also have tried what several others above had mentioned like clearing out the keychain and doing a database rebuild on the local outlook profile.
At this point I am positive the issue points to Microsoft, and they need to get this fixed today!!!!