The digital signature couldn't be validated the S/MIME control isn't available


A user had installed a Comodo mail certificate.
In the Outlook client (2010) the mail can be opened.

But in the web browser (OWA) there is an error
“This message has a digital signature. The digital signature couldn’t be validated because the S/MIME control isn’t available.”

I cannot locate an option to download or install a S/MIME control.
We are using Exchange 2010.

We are also using Nitrodesk Touchdown with Active Sync.
In this Nitrodesk app the signed mail is also not readable.

Can you help me with a solution ?

Hello Jeroen,

Welcome to the forums!
Have you read this? and does that help for OWA?

To be able to decrypt the messages you need the private key available on the Device.
More info here:

Met vriendelijke groet,