Administrators will receive notifications as follows:
"Hello Administrator,
This is an auto-generated email to notify you that Active Directory Synchronization encountered 1 failure(s) while it was processing some User(s) from your company. Following is the list of failed User(s):
John_Smith@arctera.io
Please try to run the synchronization process again. If the problem persists, please contact your client services representative."
This issue only occurs when the 'Synchronize user name' setting is set to 'User Principal Name' on the Exchange Online collector.

Work Around: Please contact support to have these users manually provisioned.
This issue is slated to be addressed in the July 2025 release of Arctera Insight.