The Mail() funciton uses the default email address only if you do not specify who the email is going to be from.
using the following at the end of the mail function will solve this problem.
Also, the mail() function will use the smtp gateway specified by you (if you are the sys admin) or most likely by your hosting company.
$to = "junkini@hotmail.com";
$subject = "wow this is cool!";
$message = "I should read www.php.net";
$from = "From: junkini@hotmail.com\r\nReply-to: junkini@hotmail.com"
mail($to,$subject,$message,$from);
Cheers.