You should be able to parse the delivery log for start and end times by transaction number. You should be able to count the number of messages that were processed between the time the event was triggered and the time you found the queue drained.

Inbound SMTP and client connections increase Spool Count, spool filtering delays delivery, then delivery process drains the spool backlog..

If you received one message for ,50 people, does the event monitor count messages or deliveries pending,?    I don't know but it might be relevant.

Currently im using the SmarterMail Linux as one of the inbound gateway for filtering to offload some load from the main server and the log does not suggest anything more than the limit i set.

If one message send to 50 people, it is counted as 1 email in the pool.

I just received an alert and checked the log and spool, still below the threshold limit.

Hello Tan,

We have not seen anything like this as the system messages in our testing environment and reports are functioning as expected. Do you have any other clues as to what is going on? If this gets more involved, you can submit a support ticket for us to dig deeper. Especially if you can replicate the issue. We are more than happy to help diagnose the problem and provide a solution.