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Symptoms
In Microsoft Word, Excel, PowerPoint, and Outlook, if you mention a user by using the @ sign followed by the user's name or email alias, the user doesn't resolve.
Cause
This issue occurs if the user is hidden from the global address list (GAL) in Microsoft Exchange Server or Exchange Online.
Resolution
To resolve this issue, use one of the methods, as appropriate for your situation.
Note: The following steps must be done by an administrator.
Method 1: Mailboxes are in Exchange Online in a non-hybrid configuration