Azure Active Directory did not register a synchronization attempt

This week I received several e-mail like the one below.

This message means, there is no synchronization from your local AADConnect to Office 365 tenant.

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Hello rvoogt,

On zondag, 03 juli 2016 23:12:57 GMT, Azure Active Directory did not register a synchronization attempt from the Identity synchronization tool in the last 24 hours for Domain [tenant.onmicrosoft.com].
You can troubleshoot this issue by running the Directory Synchronization troubleshooter on the server that has Azure Active Directory identity synchronization tools installed.

Thank you,
The Azure Active Directory Team
Do not reply to this message. It was sent from an unmonitored email account.

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The message is received because the synchronization is not working properly.

Check the synchronization :

Get-ADSyncScheduler

SyncCycleEnabled => False

This means no synchronization schedule is active !

Execute : Set-ADSyncScheduler -SyncCycleEnabled $True

And check again => SyncCycleEnabled is now True, and AADConnect will synchronize every 30 minutes.

.