Simple WordPress plugin to set the "envelope sender" correctly, so that e.g. Gmail accepts them. Just create a folder "wp-content/plugins/fix-mail-sender" and put this file there. Don't forget to activate the plugin and enjoy. Note: if you are on Hetzner, you need to create a mailbox "[email protected]" too to make it work. Don't ask why.

index.php

<?php
/**
* Plugin Name: Fix Mail Sender
* Plugin URI: https://georg-sieber.de
* Description: Sets the envelope sender correctly to the "From" header address to enable mail delivery to Gmail from managed webservers.
* Version: 1.0
* Author: Georg Sieber
* Author URI: https://georg-sieber.de
* License: free
*/

function fixEnvelopeSender($phpmailer) {
	$phpmailer->Sender = $phpmailer->From;
}
add_action( 'phpmailer_init', 'fixEnvelopeSender' );