These macros check the number of recipients on an outgoing message and if there are more than a specific number of recipients, asks what you want to do with it. The first macro lets you cancel the send, the second macro sends the message, moving the addresses to the BCC field if you click Yes, and sends as is when you click No.

To look for messages containing attachments then confirm you really want to send the attachment to everyone, see Do you want to send an attachment?

Cancel the send if more than 1 recipient

Change the recipient count (Item.Recipients.Count > 1) to the desired number.

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
  Dim intRes As Integer
  Dim strMsg As String

  If Item.Recipients.Count > 1 Then
    strMsg = "Your message " & Chr(34) & Item.Subject & Chr(34) & _
             " contains " & Item.Recipients.Count & " recipients. " & vbCrLf & vbCrLf & _
             "Do you want to send this message?"
    intRes = MsgBox(strMsg, vbYesNo + vbExclamation, "Confirm Message Send")
    If intRes = vbNo Then
      ' cancel send
      Cancel = True
    End If
  End If
End Sub

Move the addresses to the BCC field

Use this code to move addresses in the To and CC fields to the BCC field.

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
  Dim intRes As Integer
  Dim strMsg As String
  Dim i As Integer
 
  
  If Item.Recipients.Count > 1 Then
    strMsg = "Your message " & Chr(34) & Item.Subject & Chr(34) & _
             " contains " & Item.Recipients.Count & " recipients. " & vbCrLf & vbCrLf & _
             "Do you want to move them to the BCC field?"
    intRes = MsgBox(strMsg, vbYesNo + vbExclamation, "Confirm Message  Send")
    If intRes = vbYes Then
    
    For i = 1 To Item.Recipients.Count
    Item.Recipients(i).Type = 3
  Next i
    End If
  End If
End Sub