Unsubscribe URLs in emails being altered by firewalls
You may want to have a look at the Advanced Newsletter Management Extension. It allows you to configure mailing subscription preference pages in CiviCRM. The extension will then make this information available via its REST API and an external system can generate the pages and present them to your contacts.
As it does require some work to set up mailing preference forms separately from CiviCRM, it's not a quick and easy fix.