Report all received emails as spam, add a filter to get rid of the @domain.com emails, and probably add the entire service to the blacklist so it doesn't happen again. Probably get rid of any paid account on that service one might have laying around.

If the email is targeted at a domain using mail servers from any major email provider (Gmail, Outlook), the user will probably find most emails into their spam folder anyway and the sending domain will get added to the spam list automatically. Especially if the attack hits multiple users on the same email service.

Thanks, helpful. If I add a filter to ge rid of emails from the domain (ccsend.com) then I won't be able to mark them as spam. And what blacklist are you referring to? These are coming into my gmail inbox, as a result of some unprovoked online harassment 1.5+ years ago. Every day, still.