Microsoft has identified a problem with Mac Outlook 2011, part of Microsoft Office 2011 for Mac, running under OS X 10.11 (El Capitan):
https://support.microsoft.com/en-us/kb/3098396
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 mail.app 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: