Hello Stephen, its a pleasure to meet you and we're happy to have you here. You should be able to set this up in SmarterMail within Settings>Events (when logged in as a system administrator) where you'll find a variety of system categories you can use including Spool Count which will trigger based on overall outgoing message volume. Additionally, I recommend enabling Domain/User Throttling here as this should prevent too many messages going out before you're alerted to the issue. I hope this helps :-)