Private Sub TextBoxEmlPath_DragEnter(sender As Object, e As DragEventArgs) Handles TextBoxEmlPath.DragEnter
    For i As Integer = 0 To UBound(e.Data.GetFormats())
      Debug.Print(e.Data.GetFormats()(i))
    Next
    'If e.Data.GetDataPresent(DataFormats.FileDrop) Then
    '  e.Effect = DragDropEffects.Copy
    'End If
    e.Effect = DragDropEffects.Copy
  End Sub

Windows Mail Messages
FileGroupDescriptor
FileGroupDescriptorW
FileContents
Internet Message (rfc822/rfc1522)
HTML Format
System.String
UnicodeText
Text

  Private Sub TextBoxEmlPath_DragDrop(sender As Object, e As DragEventArgs) Handles TextBoxEmlPath.DragDrop
    'Dim files() As String = e.Data.GetData(DataFormats.FileDrop)
    'For Each path In files
    '  TextBoxEmlPath.Text = path
    'Next
    TextBoxEmlPath.Text = e.Data.GetData(DataFormats.Html)
    'TextBoxEmlPath.Text = e.Data.GetData(DataFormats.Text)
  End Sub

  Private Sub Form1_DragEnter(sender As Object, e As DragEventArgs) Handles Me.DragEnter
    If e.Data.GetDataPresent("Internet Message (rfc822/rfc1522)") Then
      e.Effect = DragDropEffects.Copy
    End If
  End Sub

  Private Sub Form1_DragDrop(sender As Object, e As DragEventArgs) Handles Me.DragDrop
    Dim objMemoryStream As IO.MemoryStream = e.Data.GetData("Internet Message (rfc822/rfc1522)", True)
    Dim aFileBytes(0 To objMemoryStream.Length - 1) As Byte
    objMemoryStream.Position = 0
    objMemoryStream.Read(aFileBytes, 0, (objMemoryStream.Length))
    objMemoryStream.Close()

    'если надо получить имя файла как тема_сообщения.eml -нафиг не надо
    Dim aFileGroupArray(512) As Byte
    objMemoryStream = e.Data.GetData("FileGroupDescriptor", True)
    objMemoryStream.Position = 0
    objMemoryStream.Read(aFileGroupArray, 0, 512)
    Dim fileName As System.Text.StringBuilder = New System.Text.StringBuilder(256)
    'following code extracts file name of message
    For intCount = 76 To 512
      fileName.Append(Convert.ToChar(aFileGroupArray(intCount)))
    Next
    objMemoryStream.Close()
    Debug.Print(fileName.ToString)
    '

    'create .eml file
    Dim objFileStream As IO.FileStream = New IO.FileStream("test.eml", IO.FileMode.Create)
    objFileStream.Write(aFileBytes, 0, (aFileBytes.Length))
    objFileStream.Close()
  End Sub

 Private Sub Form1_DragDrop(sender As Object, e As DragEventArgs) Handles Me.DragDrop
    Dim objMemoryStream As IO.MemoryStream = e.Data.GetData("Internet Message (rfc822/rfc1522)", True)
    Dim aFileBytes(0 To objMemoryStream.Length - 1) As Byte
    objMemoryStream.Position = 0
    objMemoryStream.Read(aFileBytes, 0, (objMemoryStream.Length))
    objMemoryStream.Close()

    'create .eml file
    Dim objFileStream As IO.FileStream = New IO.FileStream("test.eml", IO.FileMode.Create)
    objFileStream.Write(aFileBytes, 0, (aFileBytes.Length))
    objFileStream.Close()

    Dim v_Conf As String = "http://schemas.microsoft.com/cdo/configuration/"
    Dim o_Mess As CDO.Message = LoadMessageFromFile("test.eml") ' As Object
    IO.File.Delete("test.eml")
    With o_Mess.Configuration.Fields
...

  Private Function LoadMessageFromFile(ByVal Path As String) As CDO.Message 'As Object
    Dim iMsg As New CDO.Message '= CreateObject("CDO.Message")

    Dim Stm As New ADODB.Stream '= CreateObject("ADODB.Stream")
    Stm.Open()
    Stm.Type = iMsg.GetStream().Type
    Stm.Charset = iMsg.GetStream().Charset
    Stm.LoadFromFile(Path)
    Dim iDsrc As CDO.IDataSource = iMsg.DataSource ' get IDataSource interface
    iDsrc.OpenObject(Stm, "_Stream")
    LoadMessageFromFile = iMsg
  End Function