what's the point of all these random accounts creation? seems like a automated process. would it be to upload illegal stuff?

I've set registration to Require application again, don't like this bots creating accounts.

Also I'm running into email limitations. Using the Sendgrid Free plan of 100 emails per day, but if a couple of member have email notification on, I'm reaching that 100 very quickly. How do others handle the high email traffic ?