function spamcheck($field) {
    // filter_var() sanitizes the e-mail
    // address using FILTER_SANITIZE_EMAIL
    $field = filter_var($field, FILTER_SANITIZE_EMAIL);

    // filter_var() validates the e-mail
    // address using FILTER_VALIDATE_EMAIL
    if(filter_var($field, FILTER_VALIDATE_EMAIL)) {
        return TRUE;
    }
    else {
        return FALSE;
    }
}

if(isset($_REQUEST['email'])) {
    // if "email" is filled out, proceed
    // check if the email address is invalid
    $mailcheck = spamcheck($_REQUEST['email']);
    if ($mailcheck==FALSE) {
        die("<font color='red'>Your email address is invalid, please try again.</font><br/><br/><form><INPUT TYPE='button' VALUE='Back' onClick='history.go(-1);return true;'></form>");
    }
    else {//send email
        $to = "$email";
        $from = "name@mywebsite.com";
        $message = "Just a message";
        $subject = "Subject";

        mail($to, $message, $subject, "From: $from", "-f$from");
    }
}

function checkEmail($input)
{
  return filter_var($input, FILTER_VALIDATE_EMAIL);
}