From 01.11.2021, at least Outlook 2013 SP1 is required for Microsoft 365. Previously, older Outlook versions usually worked with Microsoft 365 without any problems. From 01.11.2021, however, Microsoft will now block Outlook versions older than 2013 SP1 from being used and block these versions.
Here you can find the corresponding article from Micorsoft:
Microsoft justifies the move to block old Outlook versions with improved security and stability of the newer Outlook versions (and the coolest new features).
Anyone who is currently still using very old versions of Outlook in conjunction with Microsoft 365 must therefore update or upgrade by November 1, 2021 at the latest so that the connection with Exchange Online continues to work.
