Couple thing:

1. You are not alone, this happens at a large scale across the board with companies of all sizes.

2. More than likely the abuser did not do it manually, more than likely they automated it

3. As a thoughtful business one may have rolled out all the authentication features/gates if the business picks up, as a starter the safe idea could have been to put it behind any openly available OAuth provider

Please write your blog post yourself if you expect people to read it. The LLM output is very grating.

There will always be a subset of users whose goal is to not use your service, but to arbitrage your service into the maximum value for themselves.

For example -- let's say you offer $100 in free AWS credits by signing up to your platform. Expect a malicious user to eventually come to your platform, realize they can resell those $100 in credits for $50, and start using your platform for their own gain. Unless the mechanisms you add in place to reduce fraud / second sign ups / etc is greater than the value that they are receiving ($50), they will continue.

With sites where the platform is free, the math almost always makes sense for these malicious users to eventually abuse. In this case it was leveraging the email reputation of another domain at no cost to their own (along with the added value of anyone getting phished), but on other sites it's public profiles being used for backlinks / spam, etc.