How do I decode an S/MIME e-mail message with EWS (E2010)

When I get an Email:

                PropertySet itempropertyset = new PropertySet(BasePropertySet.FirstClassProperties)
                itempropertyset.Add(ItemSchema.MimeContent);
                itempropertyset.Add(ItemSchema.Attachments);
                itempropertyset.RequestedBodyType = BodyType.Text;
                src = _Exchange.LoadPropertiesForItems(findMails, itempropertyset);

and the message was sent with S/MIME, how do I get the Text decoded?

Assume: EmailMessage em;

em.Body.Text is null