I've seen a page about Outlook 2011 having issues with OS X 10.11 at https://support.microsoft.com/en-us/kb/3098396 but haven't seen one for Exchange in general.
Thanks for the updates, until this is sorted I cannot upgrade to 10.11 because one my business emails from a partner runs on Exchange 2007 so upgrading to Office 365/2016 isn't a viable option.