Back in May of 2020 (over 3 years ago) we discontinued the legacy Exchange infrastructure and retired the legacy endpoints. At the time we recommended reconfiguring Outlook and mobile devices to use a new endpoint owa.xd.email in place of the legacy ones:
cas.rockerduck.exchangedefender.com
cas.darkwing.exchangedefender.com
cas.scrooge.exchangedefender.com
If you are still connecting to these legacy hostnames 3 years after they've been decomissioned we urge you to go through the following guide: https://exchangedefender.com/docs/exchange-setup - we have done our best to support legacy deployments for as long as possible to help our partners and busy IT departments. Keeping these legacy items in place degrades the performance and security profile of our network so for everyone's safety we have no choice but to finally discontinue the legacy names. If your Outlook is reporting an error please urgently follow the steps in the guide and update your DNS/Outlook.
We will end support for old/legacy hostnames (cas.*.exchangedefender.com) in September 2023. If you are reading this notice after September 2023, you can immediately access your email at https://owa.xd.email/owa/ and follow the guide to reconfigure your DNS, Outlook, or mobile devices.