Simple solution: give everyone address with "+suffix", and mark as junk any mail without it. For stubborn services who don't accept "+" in email addresses use

Simple solution: give everyone address with "+suffix", and mark as junk any mail without it. For stubborn services who don't accept "+" in email addresses use unique redirect aliases. (Also, IIRC, Gmail ignores dots, i.e. "j.random.user@gmail.com" and "j.r.a.n.d.o.m.u.s.e.r@gmail.com" is the same address. Chose a canonical one with a dot somewhere, and filter the rest with exception for legimate senders.)