Testing is a crucial step in any change management process, as it ensures that the change is compatible with the existing systems and does not cause any errors or disruptions. In this case, the change management team failed to test the email client patch on Windows 11 devices, which resulted in a widespread issue for the users.
Testing would have revealed the problem before the patch was deployed, and allowed the team to fix it or postpone the change.
References: 7 Reasons Why Change Management Strategies Fail and How to Avoid Them, CompTIA CySA+ CS0-003 Certification Study Guide