Microsoft released the above CU a few days ago and it comes with a new feature called AMSI, you can read all about it here:

With this CU, you need to upgrade the Schema. So let’s firstly show you the command for the admins new to Exchange that need to be run to update the Schema:

  • Setup /PrepareSchema /Iacceptexchangeserverlicenseterms

Once the above command has completed, don’t forget to Prepare Active Directory with the following command:

  • Setup /PrepareAD /Iacceptexchangeserverlicenseterms

Once the second command completes, you need to let it all replicate and then you can run the upgrade to CU21.

Hope it helps.

By edward