I run a website where you must create an account and verify via email to access many site features. Lately a bunch of the sent account confirmation emails are being marked as spam because someone or something is using real email addresses to sign up for fake accounts on my site. I know they are fake because almost all of the time the IPs are coming from Russia or have garbled usernames like 'VDLWYvAkZniTQS' and have no site activity. It's always a different IP though so just banning one at a time wouldn't help. They also sign up about 3 accounts/hr so rate limiting wouldn't really work. I'm starting to run out of ideas and my email service provider might deactivate my account because of the uptick in spam complaints. Any ideas on how to combat this?
I've tried installing both a honeypot and reCAPTCHA v2 into the sign up form but they haven't helped too much. If anything they started to use more convincing usernames now but the IPs are clearly still coming from Russia.