Mails from nonexisting domain users
A lot of SPAM looks as from "NonexistigAddres@MyDomain". Idea: Add rule for filtering mails from such senders. (Similar like Directory Harvesting for nonexisting recipients.)
This looks like functionality already present in NewSenders
Basically instead of looking at address books, we identify contacts who we have sent email to in the past.
Jiri Kryze commented
If will be possible check senders address looks like from defined our domains against of our domain users(valid addresses), there will be possibility filter out fake-senders and make rule for deletiting such mails (for example).