This demonstrates how to process an attached mailitem.

Sub msgAsAttachment()

Dim curritem As MailItem
Dim att As attachment
Dim msgInternal As MailItem
Dim attInternal As attachment
Dim tempFileName As String
Dim tempFolder As String

tempFolder = "C:\Test\" ' be careful to end with a slash
tempFileName = "dummy.msg"

Set curritem = ActiveInspector.currentItem

For Each att In curritem.Attachments

    Debug.Print att.FileName
    
    If Right(att.FileName, 3) = "msg" Then
        
        att.SaveAsFile tempFolder & tempFileName
        Set msgInternal = CreateItemFromTemplate(tempFolder & tempFileName)
        For Each attInternal In msgInternal.Attachments
            Debug.Print attInternal.FileName
        Next
        msgInternal.Delete

    End If
    
Next

Set msgInternal = Nothing
Set curritem = Nothing

End Sub