Property Value

Type: Certificate
A reference to the certificate extracted from the message signature, or a null reference (Nothing in Visual Basic) if the source message was not signed or if the current SmimeResult object was returned by Encrypt(MailMessage, CertificateCollection) method call.

You should use Verify(MailMessage, MessageVerificationFlags, CertificateStore) or DecryptAndVerify(MailMessage, MessageVerificationFlags) methods to get SmimeResult object with SignatureCertificate property set.