From mail2web.com Wiki
Why do I need to buy a domain name?
Answer: Our mail2web.com Email, Email Forwarding and Business Exchange plans require that you use a custom domain name. This allows you to establish your own unique presence on the internet as well as allowing you to choose any email address you want. If you already own a domain name that you would like to use with your mail2web.com plan, you can point it to our servers after you have finished signing up for your account.