Microsoft has announced that as of October 31st ( Moved from the 1st March ) it will be dropping TLS 1.1 Support and only allowing TLS 1.2
https://blogs.technet.microsoft.com/exchange/2018/02/09/an-update-on-office-365-requiring-tls-1-2/
What you need to do
https://support.microsoft.com/en-us/help/4057306/preparing-for-tls-1-2-in-office-365
1 ) Check for 2008 R2 and Windows 7 Devices and make sure this KB is installed : https://support.microsoft.com/en-au/help/3140245/update-to-enable-tls-1-1-and-tls-1-2-as-a-default-secure-protocols-in
Make sure this Reg key is set
2) Make sure you are not using any of these devices which do not support TLS 1.2
- Android 4.3 and earlier versions
- Firefox version 5.0 and earlier versions
- Internet Explorer 8-10 on Windows 7 and earlier versions
- Internet Explorer 10 on Win Phone 8.0
- Safari 6.0.4/OS X10.8.4 and earlier versions
3) Check your devices e.g. MFD and Phone Systems support TLS 1.2