If “Bounce On” isn’t enabled in SiteWorx->Email->Overview, messages for invalid recipients won’t be rejceted at the SMTP level. It will either silently accept and delete the message, or deliver the message to a catchall, if that is how it is setup.
the main problem was solved (with chkuser), but there is another - also connected with backscatter - how can i disable Sender callouts (Sender Verify / SAV)?