It has been announced some time ago already but a reminder is worth it as the date is approaching.
RPC over HTTP connectivity to Exchange Online is being deprecated and replaced by MAPI over HTTP on October 31, 2017.
If you are using an old version of Office/Outlook (2007, 2010 or 2013) you need to ensure you have the appropriate updates installed to continue accessing your mailbox on Exchange Online.
By the way Office/Outlook 2007 or 2010 is out of support already.
To know more about the requirements to continue accessing Exchange Online mailboxes with Outlook after October 31 go to https://support.microsoft.com/en-us/help/3201590/rpc-over-http-deprecated-in-office-365-on-october-31–2017