WEB WORKBENCH

<?php
/*
Plugin Name: Simple Email Test
Plugin URI: https://webworkbench.co.uk
Description: A simple plugin to send test emails from WordPress.
Version: 1.0
Author: Tudor Gibson
License: GPL2
*/

if (!defined('ABSPATH')) {
    exit;
}

add_action('admin_menu', 'set_add_admin_menu');

function set_add_admin_menu() {
    add_management_page(
        'Email Test',
        'Email Test',
        'manage_options',
        'simple-email-test',
        'set_email_test_page'
    );
}

function set_email_test_page() {

    $message = '';

    if (
        isset($_POST['set_send_email']) &&
        check_admin_referer('set_email_test_nonce')
    ) {

        $email = sanitize_email($_POST['set_test_email']);

        if (is_email($email)) {

            $subject = 'WordPress Email Test';
            $body = 'Congratulations! Your WordPress email system is working correctly.';
            $headers = array('Content-Type: text/plain; charset=UTF-8');

            if (wp_mail($email, $subject, $body, $headers)) {
                $message = 'Email sent successfully to ' . esc_html($email) . '.';
            } else {
                $message = 'Email failed to send.';
            }
        } else {
            $message = 'Please enter a valid email address.';
        }
    }

    echo '';
    echo 'Email Test';

    echo $message;

    echo '';
    wp_nonce_field('set_email_test_nonce');

    echo '';
    echo '';
    echo 'Email Address';
    echo '';
    echo '';
    echo '';

    submit_button('Send Test Email', 'primary', 'set_send_email');

    echo '';
    echo '';
}