To prevent MercuryS from at all accepting messages for non-existing users, uncheck "Accept mail for invalid local addresses" in MercuryS configuration. Alternatively (or additionally) one can clear the field "Delivery failure template" in Mercury core configuration / Files. To prevent Mercury from sending out delivery confirmations as well, clear "Delivery confirmation template" in the same tab.
Correction: "Delivery failure template" should not be blank, but it's OK to rename the notification file (to failure.mer.x or whatever).
<p>To prevent MercuryS from at all accepting messages for non-existing users, uncheck "Accept mail for invalid local addresses" in MercuryS configuration. Alternatively (or additionally) one can clear the field "Delivery failure template" in Mercury core configuration / Files. To prevent Mercury from sending out delivery confirmations as well, clear "Delivery confirmation template" in the same tab.</p><p>Correction:&nbsp;<span style="font-size: 13.3333px;">"Delivery failure template" should not be blank, but it's OK to rename the notification file (to failure.mer.x or whatever).</span></p><p>&nbsp;</p>