I think the following should work for you
Sub ShrinkAlert(MyMail As MailItem)
Dim olInsp As Inspector
Dim wdDoc As Object
Dim oRng As Object
Const sText As String = "CAUTION! This is an EXTERNAL email originated from outside the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe."
With MyMail
.Display
Set olInsp = .GetInspector
If Not (MyMail.Recipients Is Nothing) And Not (MyMail.sender Is Nothing) Then
'open in "Edit" mode
olInsp.CommandBars.ExecuteMso ("EditMessage")
End If
Set wdDoc = olInsp.WordEditor
Set oRng = wdDoc.Range
With oRng.Find
Do While .Execute(findText:=sText)
oRng.Text = "EXTERNAL: "
oRng.Font.Color = &HFF&
oRng.collapse 0
Exit Do
Loop
End With
.Save
.Close 1
End With
End Sub