By default, any email sent to addresses which are not listed in the control panel will be passed through the system unfiltered. Because a large percentage of junk mail is sent to email addresses that do not actually exist - i.e. firstname.lastname@example.org, email@example.com, etc. - this can put unnecessary strain on your network and your mail server by forcing it to process unwanted junk.
Please note that we can provide only very limited support for any messages that are sent to addresses that are unknown to the filter. Even if you do not want to filter out spam and viruses sent to a given address, that address should be added to the filtering system, and set as unfiltered.
Once all the valid addresses for your domain are known to the filter, we strongly recommend that the filter be configured not to accept messages sent to invalid email addresses at your domain. This can be configured via the Inbound Filtering > Spam Handling > Unknown Users tab in the control panel.