There are lists of companies with workflows like this that get used in spam campaigns. You can put any email address in and now they will get a signup notice and multiple 'hey you didnt complete our workflow!' marketing messages. Easy way to put a lot of useless messages that mostly pass spam checks in any mailbox you desire.

I recently had an incident with Chime Bank like this, where someone enrolled every public email address at my company with them. I sent them an abuse report and they told us to block their domains. Real great solution, guys.