A couple of times there have been new members signing up with no mail sent from the server. These so called members seem to be hackers. I’ve deleted them almost directly.

My thought is that, if you sign up and don’t fill all the fields, Dolphin does not send the “New member mail”

How do I prevent that all fields in the registration form must be filled out to become a member?

I'd imagine it's due to their email provider. That said, you can make fields mandatory or optional from administration, builders, profile fields, join.

The mail is send from my hostproviders server. So, how can a new member sign up without any recognizing mail from the server?