Autodiscover serve mail domain instead of hostname

I wanted my users to autoconfigure their clients with mail.theirdomain.com instead of the server hostname as I tend to move accounts around when doing server maintenance or upgrades; thus the server hostname is not always the one serving the emails, but mail.theirdomain.com always points to the correct IP as I have a script to bulk update them whenever I need to.

Long story short I replaced the $autodiscover_config part of data/web/inc/vars.inc.php with

// serve autodiscover from mail subdomain
$mail_domain = 'mail.' . implode('.', array_slice(explode('.', explode(':', $_SERVER['HTTP_HOST'])[0]), -2));

$autodiscover_config = array(
  // General autodiscover service type: "activesync" or "imap"
  // emClient uses autodiscover, but does not support ActiveSync. mailcow excludes emClient from ActiveSync.
  // With SOGo disabled, the type will always fallback to imap. CalDAV and CardDAV will be excluded, too.
  'autodiscoverType' => 'activesync',
  // If autodiscoverType => activesync, also use ActiveSync (EAS) for Outlook desktop clients (>= Outlook 2013 on Windows)
  // Outlook for Mac does not support ActiveSync
  'useEASforOutlook' => 'no',
  // Please don't use STARTTLS-enabled service ports in the "port" variable.
  // The autodiscover service will always point to SMTPS and IMAPS (TLS-wrapped services).
  // The autoconfig service will additionally announce the STARTTLS-enabled ports, specified in the "tlsport" variable.
  'imap' => array(
    'server' => $mail_domain,
    'port' => (int)filter_var(substr(getenv('IMAPS_PORT'), strrpos(getenv('IMAPS_PORT'), ':')), FILTER_SANITIZE_NUMBER_INT),
    'tlsport' => (int)filter_var(substr(getenv('IMAP_PORT'), strrpos(getenv('IMAP_PORT'), ':')), FILTER_SANITIZE_NUMBER_INT)
  ),
  'pop3' => array(
    'server' => $mail_domain,
    'port' => (int)filter_var(substr(getenv('POPS_PORT'), strrpos(getenv('POPS_PORT'), ':')), FILTER_SANITIZE_NUMBER_INT),
    'tlsport' => (int)filter_var(substr(getenv('POP_PORT'), strrpos(getenv('POP_PORT'), ':')), FILTER_SANITIZE_NUMBER_INT)
  ),
  'smtp' => array(
    'server' => $mail_domain,
    'port' => (int)filter_var(substr(getenv('SMTPS_PORT'), strrpos(getenv('SMTPS_PORT'), ':')), FILTER_SANITIZE_NUMBER_INT),
    'tlsport' => (int)filter_var(substr(getenv('SUBMISSION_PORT'), strrpos(getenv('SUBMISSION_PORT'), ':')), FILTER_SANITIZE_NUMBER_INT)
  ),
  'activesync' => array(
    'url' => 'https://' . $mail_domain . ($https_port == 443 ? '' : ':' . $https_port) . '/Microsoft-Server-ActiveSync',
  ),
  'caldav' => array(
    'server' => $mail_domain,
    'port' => $https_port,
  ),
  'carddav' => array(
    'server' => $mail_domain,
    'port' => $https_port,
  ),
);

and edited mailcow.conf with ADDITIONAL_SAN=mail.*