Send Email From Multiple SMTP in WordPress

Open your FTP client and access the theme folder of your WordPress Blog which is located in wp-content/themes/YouR Active Theme. Right-click in the functions.php and click select View/Edit.

Add these code at the end of functions.php file using your code editor.

add_action( 'phpmailer_init', 'send_smtp_email' );
function send_smtp_email( $phpmailer ) {
    $phpmailer->isSMTP();
    $phpmailer->Host       = SMTP_HOST;
    $phpmailer->SMTPAuth   = SMTP_AUTH;
    $phpmailer->Port       = SMTP_PORT;
    $phpmailer->Username   = SMTP_USER;
    $phpmailer->Password   = SMTP_PASS;
    $phpmailer->SMTPSecure = SMTP_SECURE;
    $phpmailer->From       = SMTP_FROM;
    $phpmailer->FromName   = SMTP_NAME;
}

Save the file and upload it to the server.

Access the root of the site where you will find the wp-config.php file. Right-click in the wp-config.php and click select View/Edit.

Add the line of codes in your wp-config.php just before /* That's all, stop editing! Happy blogging. */

 $servers = [
    [
        'host'   => 'smtp.example1.com',    // Address of the SMTP server.
        'port'     => '587',    // Port of the SMTP server. Options may be '25', '465', '587' or '2525'.
        'secure'   => 'tls',    // encryption to use on the SMTP connection. Valid options are '', 'ssl' or 'tls'.

        'authentication_required'   => 'true',    // set this to 'false' if your SMTP server does not require authentication
        // If 'authentication_required' above is set to 'true', the following credentials will be required
        'name'   => 'Your Name', // SMTP From name
        'username' => '[email protected]', // Username to use for SMTP authentication
        'password' => 'password', // Password to use for SMTP authentication
    ],
   [
        'host'   => 'smtp.example2.com',    // Address of the SMTP server.
        'port'     => '465',    // Port of the SMTP server. Options may be '25', '465', '587' or '2525'.
        'secure'   => 'ssl',    // encryption to use on the SMTP connection. Valid options are '', 'ssl' or 'tls'.
        'authentication_required'   => 'true',    // set this to 'false' if your SMTP server does not require authentication
        // If 'authentication_required' above is set to 'true', the following credentials will be required
        'name'   => 'Your Name', // SMTP From name
        'username' => '[email protected]', // Username to use for SMTP authentication
        'password' => 'password', // Password to use for SMTP authentication

    ],

    [
        'host'   => 'smtp.example3.com',    // Address of the SMTP server.
        'port'     => '2525',    // Port of the SMTP server. Options may be '25', '465', '587' or '2525'.
        'secure'   => '',    // encryption to use on the SMTP connection. Valid options are '', 'ssl' or 'tls'.
        'authentication_required'   => 'true',    // set this to 'false' if your SMTP server does not require authentication
        // If 'authentication_required' above is set to 'true', the following credentials will be required
        'name'   => 'Your Name', // SMTP From name
        'username' => '[email protected]', // Username to use for SMTP authentication
        'password' => 'password', // Password to use for SMTP authentication
    ],
];
$server = $servers[array_rand($servers)]; //Picks a random server (or however you want to select a server)
define( 'SMTP_FROM',   '[email protected]' );

define( 'SMTP_TO',   '[email protected],[email protected],[email protected]' );             // SMTP From email address. This can be static. 
define( 'SMTP_USER',   $server['username']);                // Username to use for SMTP authentication
define( 'SMTP_PASS',   $server['password']);                   // Password to use for SMTP authentication
define( 'SMTP_HOST',   $server['host']);                    // The hostname of the mail server
define( 'SMTP_NAME',   $server['name']);                    // SMTP From name
define( 'SMTP_PORT',   $server['port']);                    // SMTP port number
define( 'SMTP_SECURE', $server['secure']);                  // Encryption system
define( 'SMTP_AUTH',   $server['authentication_required']); // Use SMTP authentication

Save the file and upload it to the server.

Now you can add any number of SMTP servers to send mail.