Today marks the end of support for the legacy synchronisation tools which are used to connect on-premises Active Directory to Office 365 and Azure AD. Specifically Windows Azure Active Directory Sync (DirSync) and Azure AD Sync are the tools which are transitioning out of support at this time. Note also that version 1.0 of Azure Active Directory (AAD Connect) is also transitioning of support. The tools were previously marked as depreciated in April 2016.
The replacement for the older synchronisation tools is Azure Active Directory Connect 1.1. Customers must have this version of AAD Connect deployed. This is the tool which is being actively maintained, and receives updates and fixes.
Azure AD will no longer accept communications from the unsupported tools as of December 31st 2017.
If you do need to upgrade, the relevant documentation is below:
This change was announced previously in the Office 365 message centre in April 2016, via MC45036, we will be removing the Windows Azure Active Directory Synchronization feature from Office 365, beginning April 13, 2017. Azure Active Directory (AD) Connect is the best way to connect your on-premises directory with Azure AD and Office 365. Azure AD Connect is replacing DirSync and Azure AD Sync and these two older sync engines were deprecated from April 13, 2016, reaching end of support April 13, 2017. You are receiving this message because you are using DirSync, Azure AD Sync, or an Azure AD Connect installation released before 2016. After April 13, 2017, you will no longer be able to open a support case without first upgrading to Azure AD Connect.