Send WordPress email over SMTP: wp_mail with phpmailer_init, or a plugin

# option A: no plugin — create wp-content/mu-plugins/smtp.php

<?php
// wp-content/mu-plugins/smtp.php  — loaded automatically, survives theme changes
add_action('phpmailer_init', function ($phpmailer) {
    $phpmailer->isSMTP();
    $phpmailer->Host       = SMTP_HOST;
    $phpmailer->Port       = SMTP_PORT;
    $phpmailer->SMTPSecure = SMTP_PORT == 465 ? 'ssl' : 'tls';
    $phpmailer->SMTPAuth   = true;
    $phpmailer->Username   = SMTP_USER;
    $phpmailer->Password   = SMTP_PASS;
    $phpmailer->From       = SMTP_FROM;
    $phpmailer->FromName   = SMTP_FROM_NAME;
});

// wp-config.php (above "That's all, stop editing!")
define('SMTP_HOST', 'smtp.queensmtp.com');
define('SMTP_PORT', 587);
define('SMTP_USER', 'your-smtp-username');
define('SMTP_PASS', 'your-smtp-password');
define('SMTP_FROM', 'hello@yourdomain.com');
define('SMTP_FROM_NAME', 'Your Site');

// test from WP-CLI
// wp eval 'var_dump(wp_mail("you@yourdomain.com", "SMTP test", "It works"));'