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Edit your prefs.XML and set the flag "reverse_events" to true. Set the flag "v10_beta" to true. This enables v10 beta features that are otherwise disabled.
Now, in the User Manager this takes two different types of events. You need an event to accept reverse notifications, and you need an event to send out reverse notifications. You can put both of these on your default user if you like and they then apply to everyone who has not overridden that config.
Otherwise the send notifications has to go on all users who might cause a send to occur.... And the reverse needs to be on all users who you want to accept reverse notifications.
The send event notification is done via the Send Email event type like in screenshot #1. Make sure the from, to,cc,bcc are all empty. The receive event notifications is like screenshot #2.
Keep in mind reverse notifications take a bit more time and CPU power as all users have to be checked over to see if they have a VFS that matches. But if configured right, reverse notifications will then happen.