It is a certificate of the sender, to open it your code needs to verify the message is not tempered using the certificate. .Net Framework 2.0 has a SignedCms.CheckSignature Method you can use. In native code you need to call Certificate Enrollment API.
Anyway, if you need help in writing software that deal with certificates, post to the security programming forum. This forum is for general software programming questions that do not fit in any other forums.
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