From the ITS Website:
Jan. 9, 2013 - At 6:50am, IT Services began receiving reports of degraded performance with xmail and Sharepoint. The xmail problems affect users with Mac OSX running Apple Mail or Outlook 2011 or those using xmail via the web. Many users receive a pop-up regarding autodiscovery. IT Services technicians are working on the problem.
You can do some things using the web interface: http://xmail.uchicago.edu