Remove subdomain from the From address to allow mails on IONOS hosting. (See: https://torstenlandsiedel.de/2024/02/26/mailprobleme-bei-ionos-mit-subdomains/)

fix-ionos-mail.php

<?php
/**
 * Plugin Name: Fix IONOS Mail
 * Description: Remove subdomain from the From address to allow mails on IONOS hosting.
 * Plugin URI:  https://torstenlandsiedel.de
 * Version:     1.0.0
 * Author:      Torsten Landsiedel
 * Author URI:  https://torstenlandsiedel.de
 * Licence:     GPL 2
 * License URI: http://opensource.org/licenses/GPL-2.0
 */

if ( ! defined( 'ABSPATH' ) ) {
    exit; // Exit if accessed directly.
}

// Change the default "from email" by removing any subdomain
function wp_email_set_from_email( $from_email ) {
    // Use regex to match and extract the primary domain
    if (preg_match('/^(.+)@(?:[^@.]+\.)*([^@.]+\.[^@.]+)$/', $from_email, $matches)) {
        // Construct the email without the subdomain
        return $matches[1] . '@' . $matches[2];
    }
  
    // Return the original email if it doesn't match the pattern
    return $from_email;
}

add_filter( 'wp_mail_from', 'wp_email_set_from_email' );