How do you enable the mail function in XAMPP?

I am reading the book "Head First PHP & MYSQL". I am going along with the book and im in chapter 1. On page 50 it says to use the "mail()" function. When I try to use it, this error comes up "Warning: mail() [function.mail]: SMTP server response: 553 We do not relay non-local mail, sorry. in C:\xampp\htdocs\head_first\initial\alien… on line 28". I was searching online to see how to fix it and they were all saying like to change your php.ini file and your sendmail.ini file but i dont know which file that is because none of them say "sendmail.ini". Can someone please help?!?!