Microsoft announced (MC229914) that TLS 1.0 and 1.1 will no longer be supported, from Exchange Online beginning January 11th 2021.
This change will impact endpoints too.
Final Notice for disabling of TLS1.0 and TLS 1.1 Support for Exchange Online Mail Flow
We will no longer support TLS 1.0 and TLS 1.1 from Exchange Online mail flow endpoints beginning January 11th 2021. As those versions of TLS are already retired (most recently communicated in MC218794, July '20), Exchange Online customers and their partners should already be using TLS1.2 to protect SMTP connections between their email servers or devices and Exchange Online.
https://admin.microsoft.com/AdminPortal/Home?ref=MessageCenter&id=MC229914
Client side you should verify that devices are connecting to Exchange Online using TLS 1.2
About Outlook 2010/Windows 7/Windows 2008 R2 be aware that TLS 1.2 is not enabled by default, these are register modifies that you should apply ( to override problem)
Update to enable TLS 1.1 and TLS 1.2 as default secure protocols in WinHTTP in Windows
More details can be found at below blog article:
Security - TLS 1.0 & 1.1 End of Life/support for several products
https://www.alessandromazzanti.com/2020/03/security-tls-10-11-end-of-lifesupport.html
BTW be aware about this other deadline (2021, 1th November):
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.
[Other related articles]
https://jaapwesselius.com/2018/09/23/outlook-2010-disconnected-with-tls-1-2/
https://docs.microsoft.com/en-us/dotnet/framework/network-programming/tls#security-updates