pull email address from database based on query string

// http://www.gravityhelp.com/forums/topic/dynamic-population-notification-strategy
// change the 28 here to your actual form ID
add_filter('gform_notification_email_28', 'populate_admin_email_via_db', 10, 2);
function populate_admin_email_via_db($email,$lead) {

        // set the email we're going to return equal to the email that was passed
    // in from the admin settings, in case something fails
        $selected_email = $email;

        // database connection details
        $dbname  = 'databasename';
        $dbuser  = 'databaseuser';
        $dbpass  = 'databasepassword';
        $dbhost  = 'localhost or database host';
        $dbtable = 'the table that holds your ID and email';

        // the hidden accountID value from the form. it was item 4 in the $lead array for my form
        $account = $lead[4];

        // establish the connection
        $link = mysql_connect($dbhost, $dbuser, $dbpass);

        // if connection was established
        if ($link){
        // change "user_email" here to the name of the field where your email is held
        // change "user_login" here to the name of the field that holds your accountID
                $sql = "SELECT user_email FROM $dbname.$dbtable WHERE user_login='$account' LIMIT 1;";
                $result = mysql_query($sql);

                // deal with the results
                if($result) {
                        // the email address will be the one and only record returned
                        $returned_record = mysql_fetch_row($result);
                        if(!empty($returned_record))
                                $selected_email = $returned_record[0];
                }
                // close the connection if it was established
                mysql_close($link);
        }
    // return an email address one way or the other
        return $selected_email;
}