Routing to blackhole in cpanel not working

I'm trying to temporarly overcome a backscatter attack, in the meanwhile I tried to set on this domain the default address in CPanel 76.0.20 to Discard (Not Recommended). But I made a simple test, writing an email to a non existent address and I receive back a failure notice for 550 5.1.1 xxx@xxx.xx User doesn't exist. Shouldn't it work so that I receive no delivery report? Thanks