Proof of Concept Roundcube plugin for login over .well-known Mailadress to Username

hs_login.php

<?php
class hs_login extends rcube_plugin
{
    public $task = 'login';

    function init()
    {
        $this->add_hook('authenticate', array($this, 'translate_username'));
    }

    function translate_username($args)
    {
        $email = $args['user'];

        // if no @ -> its a username - do nothing
        if (strpos($email, '@') === false) {
            return $args;
        }

        list($local, $domain) = explode('@', $email);

        // if an email: check .well-known to figure out the username to the mail
        $url = "https://{$domain}/.well-known/autoconfig/mail/config-v1.1.xml?emailaddress=" . urlencode($email);

        $response = @file_get_contents($url, false, $context);

        if ($response) {
            $xml = @simplexml_load_string($response);

            if ($xml && isset($xml->emailProvider->incomingServer)) {
                foreach ($xml->emailProvider->incomingServer as $server) {
                    if ((string)$server['type'] === 'imap') {
                        $username = (string)$server->username;
                        $username = str_replace('%EMAILADDRESS%', $email, $username);
                        $username = str_replace('%EMAILLOCALPART%', $local, $username);
                        // overwrite the given "username" (a mail adress) with the found and correct username 
                        $args['user'] = $username;

                        rcube::write_log('hs_login', "Translated {$email} -> {$username}");
                        break;
                    }
                }
            }
        }
        
        // returned (modified) user credentials
        return $args;
    }

}