No sign certificate found for <...>:Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

var thumbprint = "110c7f8b2222bed66eed3583dcb4791cf988a462";
oMail.From.Certificate.FindThumbprint(thumbprint, Certificate.CertificateStoreLocation.CERT_SYSTEM_STORE_CURRENT_USER, "My");