Not running macro on automatic calculation

I now have this which works fine

Private Sub CheckBox1_Click()
Dim OutlookApp As Object
Dim Mess As Object, Recip
Recip = [L7].Value
Set OutlookApp = CreateObject("Outlook.Application")
Set Mess = OutlookApp.CreateItem(olMailItem)
With Mess
.Subject = "Task Completed"
.Body = "XXXXXX XXXXXX has completed task" & " " & Range("C7").Value
.To = Recip
.Display
.Send
End With
End Sub

But works on check and uncheck, there is no Check_Event or Uncheck_Event so how do i stop it from sending the email when its unchecked, i only want it to do it when it is checked?