Microsoft is removing support for Outlook 2007 as of October 31, 2017. RPC/HTTP, a protocol that is needed for Outlook 2007 to connect to the Exchange Online is been deprecated.
The following versions of Outlook will be required to continue using Exchange Online & Office 365 as of November 1, 2017:
Note: These are minimum requirements.
- Office 2016 plus PU.2015.12
- Office 2013 SP1 plus PU.2015.12
- Office 2010 SP2 plus PU.2015.12
Please consider checking if there are any programs or settings that are set to block the IMAP/HTTP protocol which will be new protocol in use as of November 1, 2017 onwards.
Knowledge Base Article covering on this topic is located here.
The links here should have all the answers you need, but let me know if you have any other questions.