The C# code from that link is only intended for rare occurrences where you don't have any e-mail but have a stand-alone file and its signature (a .p7s file) instead and need to validate that.

Normally, you only need to validate a signed e-mail. In that case, all you need is the code from this tutorial:

C#

// create an instance of MailMessage 
MailMessage message = new MailMessage();

// load the message from a local disk file 
message.Load("c:\\message.eml");

// validate the signature if the message is signed 
if (message.IsSigned)
{
    MailSignatureValidity result = message.ValidateSignature();
    if (result.Valid)
        Console.WriteLine("The signature is valid.");
    else 
        Console.WriteLine("The signature is not valid.");
}
else 
{
    Console.WriteLine("The message is not signed.");
}

VB.NET

'create an instance of MailMessage 
Dim message As New MailMessage

'load the message from a local disk file 
message.Load("c:\message.eml")

'validate the signature if the message is signed 
If message.IsSigned Then 
    Dim result As MailSignatureValidity = message.ValidateSignature()
    If result.Valid Then 
        Console.WriteLine("The signature is valid.")
    Else 
        Console.WriteLine("The signature is not valid.")
    End If 
Else 
    Console.WriteLine("The message is not signed.")
End If