Implementing Multithreaded Bulk Email Sending with PHP and Swoole

In the internet era, bulk email sending is essential for both personal and enterprise workflows, but traditional single‑threaded PHP scripts are slow and inefficient when handling large volumes.

Swoole Overview

Swoole is a C‑based PHP extension that provides multi‑process, multi‑thread, asynchronous I/O, and coroutine capabilities, enabling high‑concurrency, high‑performance server applications. Unlike standard PHP, which processes requests sequentially, Swoole can handle other tasks while waiting for I/O operations.

Implementing Multithreaded Bulk Email Sending

The following code demonstrates how to use Swoole coroutines to send emails concurrently. It defines SMTP server settings, a list of recipient addresses, and creates a coroutine for each email to perform the SMTP handshake, authentication, and message transmission.

<code>// Define email information
$smtp_server = "smtp.xxx.com";
$smtp_username = "[email protected]";
$smtp_password = "xxx";
$smtp_port = 25;
$smtp_ssl = false;
$email_from_address = '[email protected]';
$email_from_name = 'xxx';
$email_subject = 'xxx';
$email_body = 'xxx';

// Define email list
$email_list = array(
    '[email protected]',
    '[email protected]',
    '[email protected]'
);

// Create coroutine
go(function () use ($smtp_server, $smtp_username, $smtp_password, $smtp_port, $smtp_ssl, $email_from_address, $email_from_name, $email_subject, $email_body, $email_list) {
    foreach ($email_list as $email_to_address) {
        $client = new Swoole\Coroutine\Client(SWOOLE_SOCK_TCP);
        $ret = $client->connect($smtp_server, $smtp_port, $smtp_ssl);
        if (!$ret) { echo "Error: " . $client->errMsg; return; }
        $response = $client->recv();
        if (strpos($response, '220') === false) { echo "Error: " . $response; $client->close(); return; }
        $ret = $client->send("helo smtpserver\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $response = $client->recv();
        if (strpos($response, '250') === false) { echo "Error: " . $response; $client->close(); return; }
        $ret = $client->send("auth login\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $response = $client->recv();
        if (strpos($response, '334') === false) { echo "Error: " . $response; $client->close(); return; }
        $ret = $client->send(base64_encode($smtp_username) . "\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $response = $client->recv();
        if (strpos($response, '334') === false) { echo "Error: " . $response; $client->close(); return; }
        $ret = $client->send(base64_encode($smtp_password) . "\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $response = $client->recv();
        if (strpos($response, '235') === false) { echo "Error: " . $response; $client->close(); return; }
        $ret = $client->send("mail from:<{$email_from_address}>\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $response = $client->recv();
        if (strpos($response, '250') === false) { echo "Error: " . $response; $client->close(); return; }
        $ret = $client->send("rcpt to:<{$email_to_address}>\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $response = $client->recv();
        if (strpos($response, '250') === false) { echo "Error: " . $response; $client->close(); return; }
        $ret = $client->send("data\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $response = $client->recv();
        if (strpos($response, '354') === false) { echo "Error: " . $response; $client->close(); return; }
        $ret = $client->send("from: {$email_from_name}<{$email_from_address}>\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $ret = $client->send("to: <{$email_to_address}>\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $ret = $client->send("subject: {$email_subject}\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $ret = $client->send("{$email_body}\r\n.\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $response = $client->recv();
        if (strpos($response, '250') === false) { echo "Error: " . $response; $client->close(); return; }
        $ret = $client->send("quit\r\n");
        if (!$ret) { echo "Error: " . $client->errMsg; $client->close(); return; }
        $client->close();
    }
});
</code>

Code Explanation

The script first sets SMTP credentials and a recipient list, then creates a coroutine for each address. Inside each coroutine, it establishes a TCP connection to the SMTP server, performs the SMTP handshake (HELO, AUTH LOGIN), sends the MAIL FROM, RCPT TO, DATA commands, transmits the email headers and body, and finally issues QUIT before closing the connection.

Advantages

Using Swoole with PHP provides an asynchronous I/O model that dramatically improves concurrency, a multithreaded approach that distributes email‑sending tasks across multiple threads, and lightweight coroutines that reduce context‑switch overhead, resulting in faster, more reliable bulk email delivery.