When you send a signed email message in a Microsoft Exchange Server 2010 environment, you cannot view the message delivery report by using Microsoft Office Outlook or Microsoft Outlook Web App (OWA). Additionally, you receive the following error message on the Delivery Reports page:
Delivery Reports don’t support tracking messages of this type.
This issue occurs because the delivery report does not support the IPM.Note.SMIME.MultipartSigned message class.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.