When you try to create an inbox rule by using Outlook on the web (formerly known as Outlook Web App [OWA]) or by using the Exchange Management Shell, you receive the following error message:
There are multiple recipients matching the identity "<User>". Please specify a unique value
This problem occurs if multiple mailboxes or mail contacts have the same display name. Exchange examines the Display name field or the displayName attribute when it creates a rule.
Change the display name in the Exchange admin center or in Active Directory Users and Computers so that the name doesn't conflict with an existing mailbox or mail contact.
To avoid this problem, use the user's Simple Mail Transfer Protocol (SMTP) address instead of the display name when you create rules.