Exchange 2013 CU9 -MessageCopyForSentAsEnabled Error: A parameter cannot be found...

So, you just updated Exchange 2013 to CU9 and you're excited that Microsoft brought back the ability to save Sent Items in Shared Mailboxes (why they got rid of that I don't know).

When you run the cmdlet Set-Mailbox "shared mailbox name" MessageCopyForSentAsEnabled $True you get an error stating: "A parameter cannot be found that matches parameter name 'MessageCopyForSentAsEnabled'.

The problem is: ADPrep is needed to take advantage of the new features in CU9, and while everyone thought the installer would run PrepareAD, just like all previous installers, it doesn't.

You have to run it separately...luckily, you can run it after the CU9 install (you DO NOT have to reinstall), and you can run it during business hours as it has little impact on replication traffic and will not affect users.

To run ADPrep, open an Elevated Command Prompt, and cd to the CU9 installation directory:

cd C:\Temp\CU9

**Note** C:\Temp\CU9 is where my installer is saved, adjust accordingly.

Then run the setup command:

Setup.exe /PrepareAD /IAcceptExchangeServerLicenseTerms

It happens pretty quickly, and once it's done check your replication to make sure it's clean by running:

repadmin /showrepl

Give it a few minutes depending on your organization size and try to run the MessageCopyForSentAsEnabled cmdlet again:

Now Sent Items will go to the Share Mailbox Sent Items as they should've in the first place.