Foiling a Protohackers email spam bot

I've been receiving lots of "Undeliverable Mail Returned to Sender" lately for Protohackers signup attempts. Protohackers login is via a "magic link", so attempting to sign up or log in results in sending an email. But none of the email body is user-controlled, so I don't really see the logic in abusing this form to spam people. One email address has been put in over 100 times over the last 3 days, and I received "Undeliverable Mail Returned to Sender" each time, because it is a GMail address that doesn't exist. What's the logic in this? Most of the email addresses did exist however and presumably the spam either reached them or was filtered by GMail. Some ideas I can think of: someone griefing particular users by bombarding them with signup spam for hundreds of services they don't use someone trying to get me specifically banned from GMail by making me send lots of unsolicited emails to GMail addresses some grey-hat chaos-monkey type operation trying to nudge all website operators…