Many Hosting companies do 'Mail-Parking' plans, which is exactly what i believe you are reffering to. They are designed to give you the Mail features you get with hosting plans (i.e POP3 Accounts, WebMail), but without the actual hosting. Obviously the plan works out cheaper than buying one with hosting.
IF you wish, check out:
http://www.liquidsix.com/hosting/mail.asp. YES this is a company i am part of, no intensional advertising meant, the link however will give a better demonstration of 'Mail Parking' and what you get and don't get with it.