Here is how you should set up the $from variable to get it to populate the header correctly:
$from = "From: $username <$useraddress>\r\n";
Notice you can put the name just before the actual e-mail address. This would put their actual NAME into the "From" field and replies would be correctly sent to the e-mail address provided right after the name.
Also, to have the script redirect to a "success" or error page depending on if they mail was sent successfully:
If (mail( $to, $subject, $message, $from)) {
header("Location: email_sent.php");
} else {
header("Location: mail_error.php");
}