How does this affect me?: Beginning October 31, 2017, Outlook for Windows clients using RPC over HTTP will be unable to access their Exchange Online mailbox.

What do I need to do to prepare for this change?: The necessary action depends on the version of Outlook in use in your organization.

 
If you are using Outlook 2007 or earlier, you need to upgrade. Outlook 2007 does not contain support for the MAPI/HTTP protocol. We encourage you to update to the Office 365 ProPlus subscription, or access Outlook via the web browser (which is included in your current subscription plan).

 
Outlook 2010-2016 customers will need to ensure their version of Outlook for Windows is set up to support MAPI/HTTP. At a minimum, you should ensure you have installed the December 2015 update. 

 
Lastly,  ensure your Outlook clients are not using a registry key to block MAPI/HTTP.