We have received several tickets regarding an expired certificate for ExchangeDefender legacy endpoints (darkwing.exchangedefender.com, rockerduck.exchangedefender.com, etc). If you are still seeing these errors it's likely because your IT guy has been asleep for years and hasn't updated DNS records that have been phased out from production in early 2020.
Please follow the following guide to setup your DNS correctly: https://www.exchangedefender.com/docs/exchange-setup
tl;dr; Make sure your OWA bookmark is at https://owa.xd.email/owa and that your autodiscover cname is pointed to autodiscover.xd.email. If you do not have the technical resources in house to make DNS changes/etc, please contact us at https://www.exchangedefender.com/help and we'll do our best to assist.