Forum Receiving massive amount of bouncing emails from spammer registrations
 

I have been getting thousands of bouncing activation emails per day in the last few days. These are mostly registrations using dubious email addresses with meaningless names or from websites with typical spam content. All the IP addresses seem to be different (I haven't checked all of them of course as there are thousands). I have tried to change the human verification system (reCAPTCHA, custom question), but it doesn't make any difference. What can be done about it ?