print_r($select_data);

//Array ( 
//[0] => Array ( [username] => user [mail] => user@gmail.com //[password] => userpassword [newsletter] => NULL) 
//[1] => Array ( [username] => user [mail] => user@gmail.com //[password] => userpassword [newsletter] => NULL) 
//)

foreach ($select_data as $valuee) {
echo $valuee['mail'];
echo $valuee['password'];
};
//user
//userpassword
//user
//userpassword