@morocco
If you have an problem with the number of emails allowed or get marked as spam than most likely it would be an issue with your host or hosting plan, not dolphin specific. You need a higher profile plan that can handle all of that.

ddos is on your end or hosts end too. Nothing to do with dolphin. Shared hosting will block it to some extent, but if it happens regularly or becomes a habit they will not tollerate it for too long. You need more control, perhaps vps or dedicated, and then see more it will be up to you rather than the host to take care of it. Otherwise you can pay for someone else to handle it. They often do not handle it that well. Depends on what you are willing to pay for that option. A few dollars per month usually doesn't take care of it. For a few hundred it probably will, but most do not want to spend that kind of cash.

Most of what you suggest is on your host side and nothing to do with dolphin. So you might see what you can do based on what your type of hosting allows, or upgrade to a plan that gives you more control over it.

None of what you mention seems to be a security hole, but a hosting allowed or capped type of thing. They have certain limits which you have reached or not allowed. So most likely you need a bigger, more powerful, or a plan that allows more plain and simple. You have out grown your host.