Comments by cybertaz (Top 2 by date)

The best I can work out is when the Email is about to be sent the mailitem To becomes a read only property. Any changes to the To generates an email that has a blank recipient list on the sent email.
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Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim e As Outlook.MailItem
Dim recip As Outlook.Recipient
Dim Myrecip As Outlook.Recipient
Dim myString As String
Set e = Item
For Each recip In e.Recipients

If InStr(recip.Address, "XXXXXXXXXX.com") > 1 Then

Set Myrecip = e.Recipients.Add(Replace(recip.Address, "XXXXX.com, "xxx-xxx.com"))
recip.Delete

End If
Next

End Sub