Smells like „product engineering“. So a product or an engineering lead gets a task to reduce risks of specific abuse by preventing someone from sending email from yahoo or web.de clone. As a quick solution they add this filter without „overthinking“ it. The impact is low, a few customers in a million, so its stupidity gets unnoticed and, once first complaint reaches them, quietly deprioritized to death. Removing it is cheap: the justification for taking that work is likely the show stopper. Google is an old large corp that hires and fires at a scale. Owning removal of abuse filter to increase revenue by Planck-sized amount is an impossible thing.