Questions

If you see this error message when loggin in to your site's control panel you need to enter your email address there.
This is used to send message related to your Site, such as "Your Mailbox is full" etc.
Is is generally a good idea to use address unrelated to your site, such as an email address provided by your ISP (something like me@myinternetprovider.com), as long as you check it regularily, or perhaps an address of a friend you are in constant contact with.