I know I *can* use Redemption to do this, but I've been able to do
everything else I needed to without using Redemption, so I'm hoping I
can overcome this last obstacle without having to use Redemption.
According to the docs:

http://msdn2.microsoft.com/en-us/lib...opertyaccessor

It looks like the PropertyAccessor is where I need to be to add this,
but the code below isn't working:

Private Sub Custom_ItemSend(ByVal oItem As Object, ByRef Cancel As
Boolean) Handles Application.ItemSend
Try
oItem.PropertyAccessor.SetProperty("http://
schemas.microsoft.com/mapi/string/{00020386-0000-0000-
C000-000000000046}/x-testheader", 12345)
oItem.Save()
Catch ex As Exception
Debug.Print(ex.Message)
End Try
End Sub

The message is sent and doesn't throw an exception but the x-header
isn't there. Any ideas here?