Information
You can enable this setting to allow users to download the S/MIME control to read and create signed and encrypted messages.
Rationale:
S/MIME uses digital signatures and encryption to protect against several classes of attacks including eavesdropping, impersonation, and tampering.
Solution
To implement the recommended state, execute the following PowerShell cmdlet:
Set-OWAVirtualDirectory -identity 'owa (Default Web Site)' -SMimeEnabled $true