Just saw this important blog issue from Microsoft.
On April the 15th Microsoft is renewing the TLS Certificate used by Office 365.
This means that hybrid mail flow may be broken for users who do not take appropriate action.
This fix is quite simple. All on-prem servers used for hybrid need to be updated to Exchange 2013 CU9 or later and the Exchange hybrid wizard needs to be run again.
You can find the latest version of the hybrid wizard over here
You can find the full Microsoft blog regarding this issue over here