Microsoft has identified a problem with Mac Outlook 2011, part of Microsoft Office 2011 for Mac, running under OS X 10.11 (El Capitan):

Please note: The problem does not affect Mac users who upgrade to OS X 10.11, and then read their Exchange email using OS X’s built-in or who access Exchange via the web.

The fix is to ensure MS Office 2011 is updated to the latest version, 14.5.6. For more information about this update:

3098229 Microsoft Office for Mac 2011 14.5.6 update