The program
Mail that comes built into OS X is an application for reading and writing email.
Your .mac email address is an account where you can receive emails.
These are two different things.
Mail is your kitchen table where you can sit down to read your mail and write letters. The .mac account is your street address, where people send your mail.
If you don't want to pay for the .mac account after the free trial, there are plenty of other options for getting a new "street address" ... you can use
Mail with almost all of them. One example is Gmail, which is a free email account you can get from Google. You can get an account by going to
www.gmail.com. You can then access your Gmail account in
Mail by following
these easy steps.
First step would be to contact the company that provides your internet access (cable company, phone company, AOL, whatever). You probably have a free email account through that company as part of your internet access fee each month.