From November 1, 2021, Microsoft requires Outlook 2013 Service Pack 1 (with fixes) as the minimum client version to connect to Exchange Online. Given all the publicity about attacks against the on-premises version of Exchange earlier this year, it’s a wonder why organizations continue to allow people to use outdated client software to connect to Exchange Online. In any case, the drop-dead date is November 1. If you have any old Outlook 2007, Outlook 2010, or Outlook 2013 (before SP1) clients, it’s time to start upgrading.