In the past, using outdated clients with Microsoft 365 services was a matter of being in an unsupported state with all the risks that go with it. This meant, that things might not work or you could experience reduced functionality. Overall, things usually kept working with a few consequences or glitches here and there.

A change in this stance was announced today per Message Center bulletin MC229143:

To ensure that we meet performance expectations, we are updating the supported versions of Outlook for Windows that can connect to Microsoft 365 services. Effective November 1, 2021, the following versions of Outlook for Windows, as part of Office and Microsoft 365 Apps, will not be able to connect with Office 365 and Microsoft 365 services.

This means, running old unsupported Outlook versions will go from “possible performance and reliability issues” to becoming actively blocked. This block will apply to these versions in the table below; as indicated, these builds were surpassed somewhere in 2017:

While it is true that many customers are stretching the lifetime of their on-premises products beyond their support dates, I’m sure – apart from functionality and management options – performance and reliability is becoming more and more of an issue.

Finally, when this notice concerns you, it means you have not been updating your clients for at least 3 years. So, get planning, as you have around 11 months to update your clients. It also may affect any existing plans of moving to Exchange Online in the future, as getting your client-base in a supported state will become a requirement, and will no longer be a serious recommendation.