Outlook 2011, OS X El Capitan and the Pinwheel of Patience
If you’re planning to upgrade to OS X El Capitan and you use Outlook 2011 to get email from Microsoft Exchange, you may want to delay upgrading. On El Capitan, connecting to Exchange email servers causes Outlook 2011 to freeze and display a beachball cursor.
Update 10-7-2015: Microsoft has released Microsoft Office for Mac 2011 14.5.6 Update, which resolves the problem with Outlook freezing. To get the update, please use the Microsoft AutoUpdate application or download it manually from the link below:
The issue appears to only affect Outlook 2011 when configured to access Exchange servers. When set up with only IMAP accounts, Outlook 2011 does not appear to display this behavior.
Microsoft is aware of the issue and has posted a knowledgebase article about it: