WordPress/wp-includes/class-smtp.php

<?php

/**
 * The SMTP class has been moved to the wp-includes/PHPMailer subdirectory and now uses the PHPMailer\PHPMailer namespace.
 */
_deprecated_file( basename( __FILE__ ), '5.5.0', WPINC . '/PHPMailer/SMTP.php', __( 'The SMTP class has been moved to the wp-includes/PHPMailer subdirectory and now uses the PHPMailer\PHPMailer namespace.' ) );
require __DIR__ . '/PHPMailer/SMTP.php';