This seems like it would take a lot of effort to mildly annoy someone. And the only way it can be automated is if the service doesn't protect itself against au

This seems like it would take a lot of effort to mildly annoy someone. And the only way it can be automated is if the service doesn't protect itself against automated user sign-ups, which they will either start doing once someone really takes advantage of it, or will result in their domain being categorised as spam once they start sending lots of sign-up emails (either by the user or gmail in general).