Answer
Emails sent from custom events pass through the NOTIFICATIONS queue, but since many other application message types also flow through that same queue with the same common attributes, the practical way to identify which messages came specifically from a given custom event is by checking the Sender field — for messages triggered by a custom event, this field holds the ID of that specific custom event. Keep in mind that application messages are usually cleaned up by a scheduled database task after some retention period, so for anything beyond recent history, you'll need to have archived the relevant messages ahead of time using the Application Message Archive functionality, rather than expecting to find old messages still sitting in the live queue.