Not running macro on automatic calculation

I suggest you replace your checkboxes with Forms controls and then assign one macro to all of them using something like this:

Sub Mail()
Dim OutlookApp As Object
Dim Mess As Object, Recip
Dim lRow as long
Dim cb as Checkbox
set cb = activesheet.checkboxes(application.caller)
if cb.Value <> 0 then
lRow = cb.topleftcell.row
Recip = cells(lRow, "L").Value
Set OutlookApp = CreateObject("Outlook.Application")
Set Mess = OutlookApp.CreateItem(olMailItem)
With Mess
.Subject = "Task Completed"
.Body = "Daniel Schofield has completed task" & " " & "-" & " " & Cells(lRow, "C").Value
.To = Recip
.Display
.Send
End With
end if
End Sub