Domino: how not to send delivery failures to the internet

A customer of mine made me a request: he doesn’t want a delivery failure with the indication that the user does not exist in the Directory to be sent back when someone outside his domain sends a mail to a non-existing address.

By default this non delivery report is sent, but you can easily change the behavior of Domino.
Go in the configuration document of the server, then  Router/SMTP -> Advanced -> Controls. Hold undeliverable mail: set to enabled.

This is the explanation of that setting
Enabled – When the Router cannot deliver a message, it leaves the message in MAIL.BOX rather than generate a delivery failure report
Disabled – (default) When the Router cannot deliver a message, it generates a delivery failure report