Hi @ Kelvin Shee,
Just wondering if you created a remote mailbox on-premises for this recipient?
This issue occurs if the user was created in Microsoft 365 and the on-premises Exchange environment doesn't have objects for the user to reference.
You can refer to this link to create a remote mailbox on-prem, and then test whether sending mail is successful after directory synchronization:
NDR error 550 5.1.10 RESOLVER.ADR.RecipientNotFound - Exchange | Microsoft Learn
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