New User Registration Every 3-5 Minutes???

Starting a day or two ago I started getting a New User Registration email from my WP blog every 1-5 minutes. I thought maybe a guest posting service had sat someone down to create some accounts but it is continuing and went all through the night…

Normal? not.

So I don’t get the value of this.

I had the Anyone can register checked but I still don’t get the value.

I see articles in my backoffice from guest posters but I have to approve them before they go out.

Does anyone know the reason someone would do this. I unchecked the anyone can register so that ought to fix it.