Office 365 is Moving to TLS 1.2 for encryption
Office 365 is moving to TLS 1.2 to provide best-in-class encryption, and to ensure service is more secure by default, Office 365 is moving their online services to Transport Layer Security (TLS) 1.2+.
Beginning March 1, 2018, Office 365 will be removing support for TLS versions less than 1.2 for their online services. All client-server and browser-server combinations must use TLS 1.2 or later protocol versions to be able to connect without issues to Office 365 services. This may require certain client-server and browser-server combinations to be updated.
If you are using any on-premises infrastructure for hybrid scenarios or Active Directory Federation Services, make sure that these infrastructures can support both inbound and outbound connections that use TLS 1.2.
Microsoft recommends that you proactively address weak TLS usage by removing TLS 1.0/1.1 dependencies in your environments and disabling TLS 1.0/1.1 at the operating system level where possible.