We have a strange issue with one of our outlook clients (outlook 2016).

Emails are been sent to the user, they appear in the users iPhone Mail and on OWA but do not appear in the users outlook inbox.

On OWA the email is shown but with an error "This message has a digital signature. The digital signature couldn't be validated because the S/MIME control isn't available."

Why does Outlook 2016 client not show this email, if the email is forward to the users own email for OWA it appears in the outlook client.

Any advice on this welcomed