// To use the code below, import MailBee namespaces at the top of your code.
using MailBee;
using MailBee.Mime;

// The actual code (put it into a method of your class).
int innerMessageCount = 0;
MailMessage msg = new MailMessage();
msg.LoadMessage(@"C:\Docs\TestMail.eml");
foreach (MimePart part in msg.MimePartTree.GetAllParts())
{
    if (part.IsMessageInside)
    {
        innerMessageCount++;
    }
}
Console.WriteLine("Number of inner messages: " + innerMessageCount.ToString());

' To use the code below, import MailBee namespaces at the top of your code.
Imports MailBee
Imports MailBee.Mime

' The actual code (put it into a method of your class).
Dim innerMessageCount As Integer = 0
Dim msg As New MailMessage
msg.LoadMessage("C:\Docs\TestMail.eml")
For Each part As MimePart In msg.MimePartTree.GetAllParts()
    If part.IsMessageInside Then
        innerMessageCount = innerMessageCount + 1
    End If
Next
Console.WriteLine("Number of inner messages: " & innerMessageCount.ToString())