Ask HN: Somebody tries to wreck my AWS SES complaint rate

I run a weekly newsletter with ~5000 subscribers.

Sending happens via AWS SES (Simple Email Service) and I implemented all necessary steps to handle bounces and complaints and had no issues for like two years.

For the last two weeks however, I see bots signing up for my newsletter for random addresses and getting sometimes an immediate complaint via AWS, hence the complaint rate is going up and is already above the warning limit.

I am not sure on the motivation there - no spamming is possible via the signup form. Obviously the CSRF protection does not help as the bots pull a page before submitting the signup form.

Has anyone got a clue what the motivation to do that might be? What can they learn from their actions?