The programmers' job would be wonderful if it wasn't for the users.
However on further reflection, you could check if the message was being sent by CC or BCC and check with the user to confirm. You could also check whether there were multiple recipients in the TO line, which I have added here. If there is only one recipient in the TO line there is no prompt.
Option Explicit
Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
'Graham Mayor - https://www.gmayor.com - Last updated - 29 Jul 2019
Dim recips As Outlook.Recipients
Dim recip As Outlook.Recipient
Dim strList As String, strTo As String
Dim oColl_To As Collection, oColl_Copy As Collection
Dim lngAsk As Long, i As Long
Set recips = Item.Recipients
Set oColl_To = New Collection
Set oColl_Copy = New Collection
For i = 1 To recips.Count
Set recip = recips(i)
If recip.Type = olCC Or recip.Type = olBCC Then
oColl_Copy.Add recip.Address
Else
oColl_To.Add recip.Address
End If
Next i
If oColl_Copy.Count > 0 Or oColl_To.Count > 1 Then
strTo = ""
For i = 2 To oColl_To.Count
strTo = strTo & oColl_To(i) & vbCr
Next i
strList = ""
For i = 1 To oColl_Copy.Count
strList = strList & oColl_Copy(i) & vbCr
Next i
lngAsk = MsgBox("You are copying the current message to:" & vbCr & vbCr & strTo & strList & vbCr & _
"Are you sure you intended to do that?", vbYesNo)
If lngAsk = vbNo Then Cancel = True
End If
lbl_Exit:
Set recip = Nothing
Set recips = Nothing
Set oColl_To = Nothing
Set oColl_Copy = Nothing
Exit Sub
End Sub