Use MAIL() that'll work. That's supposed to be configured with the servers mailing system. To process a form you do something like this. Put the following in mail.php
<?php
$extra = "Reply-To: $email\r\n";
$mess = "E-Mail : $email\nName : ".$name."\nIP:".$REMOTE_ADDR."\n\nMessage:\n--------\n".$message;
mail("users@theirmail.com", $subject, $mess,
"From: Whatever Website\r\n"
."Reply-To: $email\r\n");
echo("Thanks. Your message has been sent.<br><br><a href=\"".$HTTP_REFERER."\">Go Back.</a>\n");
?>
This is just a contact form. Of course you'll get the picture... Besides if you need help just supply more detailed information and what your form is doing.