Use Notification History auditing
Summary
JEMH can save details of notifications that have been sent by JEMH - the original source of the notification, and its recipients. An example of its usage would be validating that mails were sent to particular parties for particular actions.
Filtering this view (e.g. by Issue / user / recipient) is an open feature https://thepluginpeople.atlassian.net/browse/JEMH-7101
Viewing notification recipients
Under the Action ...
menu for each audit item, you will find a View Recipients
option. This displays a list of all recipients of notifications for an event.