var last_response_len = 0;
    $.ajax("mailer.php", {
        xhrFields: {
            onprogress: function(e) {
                var this_response,
                    response = e.currentTarget.response;

                if(!!last_response_len === false) {
                    this_response = response;
                    last_response_len = response.length;
                } else {
                    this_response = response.substring(last_response_len);
                    last_response_len = response.length;
                }

                console.log("pis data: " + this_response);
            }
        }
    }).done(function(data) {
        console.log("end : " + data);
    }).fail(function(data) {
        console.log("error: ", data);
    });

    console.log("request send.");

 $acceptors = array(
    'mail1@box.loc',
    'mail2@box.loc',
    'mail3@box.loc',
    'mail4@box.loc',
   //очень большой массив 10 000 элементов
);


// Используетм костыль:
ob_end_flush();

foreach ($acceptors AS $acceptor) {
    echo 'SENDED TO: ' . $acceptor . "\r\n"; flush(); // Явный сброс буфера
    usleep(1000); 
}

echo 'COMPLETED!';