For years, I had a website hosted by network solutions and found their tech support good. I'd call their tech support and see if they can do anything.
If I read your message right, it's not that you are getting 5000 spams a day, but that someone else is sending spam listing your email as the sender, and you're getting 5000 replies for failed deliveries or other unable to deliver issues? Yikes. I don't know how you fix that. Maybe you have to change your email.
Maybe you can find some ideas here.
http://www.wikihow.com/Stop-Spam