Decrypt a mail with MailItem of Outlook and Certificates of VerSig

Hi,
i have one a problem, i am develop a application who read a mail from
Exchange Server. my application using Primary Interop Assemblies (PIA) for
connect with Exchange Server to simulate a Outlook.
in this moment i can read all mail but when a read a mail with Digital ID
my application send a exeption and do not achieve download he attachment.

i put my code.
if someone can help me
thank you very much beforehand..

Outlook.Application oApp = new Outlook.Application();
Outlook.NameSpace...

Decrypt a mail with MailItem of Outlook and Certificates of VerSig