How to stop returning User unknown in virtual alias table

Hi,

Newbie to the systems.

Firstly can I say just how brilliant I think the webmin and virtualmin systems are.

Please can someone tell me how I can stop the system replying with “User unknown in virtual alias table” when sending to a username not hosted by the server.

Spammers can use this returning message to spam unsuspecting hosts.

I would like emails sent to non-existent users of any of the domains to be dropped, without a reply.

I did a quick search of this here, but its hard to find stuff when you don’t know what the proper names for things are!!!

Many thanks,

Dave