Public Sub ReplyMail()
  Dim obj As Object
  Dim oReply As Outlook.MailItem
  ' Text, der dem Betreff vorangestellt werden soll
  Const FIX_SUBJECT As String = "Automatische Antwort von x: "

  ' Verweis auf das aktuell geöffnete oder selektierte Element:
  Select Case True
  Case TypeOf Application.ActiveWindow Is Outlook.Inspector
    Set obj = Application.ActiveInspector.CurrentItem
  Case Else
    With Application.ActiveExplorer.Selection
      If .Count Then
        Set obj = .Item(1)
      End If
    End With
    If obj Is Nothing Then
      Exit Sub
    End If
  End Select

  If TypeOf obj Is Outlook.MailItem Then
    Set oReply = obj.Reply
    If InStr(1, oReply.Subject, FIX_SUBJECT, vbTextCompare) = 0 Then
      oReply.Subject = FIX_SUBJECT & oReply.Subject
    End If
    oReply.Display
  End If
End Sub