Is there a way I can bounce all email addressed to unknown users?
Yes, you can do this by turning off the "catch-all" rule found in your mailbox settings:
Log into the Mailboxes & addresses section of OnSite. Below the list of email addresses, find the 'No catch-all' button. Mark that button and then click submit.
All mail sent to an address that does not have an explicit delivery rule will now be returned to sender.
