composer require phpmailer/phpmailer

<?php

use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\SMTP;
use PHPMailer\PHPMailer\Exception;

require 'vendor/autoload.php';

$mail = new PHPMailer(true);

try {
    // Server settings
    $mail->SMTPDebug = SMTP::DEBUG_OFF;
    $mail->isSMTP();
    $mail->Host       = 'smtp.lettr.dev';
    $mail->SMTPAuth   = true;
    $mail->Username   = 'lettr';
    $mail->Password   = 'your-api-key';
    $mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;
    $mail->Port       = 587;

    // Recipients
    $mail->setFrom('you@example.com', 'Your Name');
    $mail->addAddress('recipient@example.com', 'Recipient Name');
    $mail->addReplyTo('reply@example.com', 'Reply Address');

    // Content
    $mail->isHTML(true);
    $mail->Subject = 'Hello from Lettr';
    $mail->Body    = '<p>Welcome to <b>Lettr</b>!</p>';
    $mail->AltBody = 'Welcome to Lettr!';

    $mail->send();
    echo 'Message has been sent';
} catch (Exception $e) {
    echo "Message could not be sent. Mailer Error: {$mail->ErrorInfo}";
}

$mail->addAttachment('/path/to/file.pdf', 'document.pdf');
$mail->addAttachment('/path/to/image.jpg', 'photo.jpg');

$mail->addCC('cc@example.com');
$mail->addBCC('bcc@example.com');

$mail->SMTPSecure = PHPMailer::ENCRYPTION_SMTPS;
$mail->Port       = 465;

$mail->SMTPDebug = SMTP::DEBUG_SERVER;