Fix NDR error 550 5.1.10 in Exchange Online - Exchange
https://learn.microsoft.com/en-us/exchange/troubleshoot/email-delivery/ndr/fix-error-code-550-5-1-10-in-exchange-online
WebMay 1, 2010 · In the Microsoft 365 admin center, choose Users to go to the Active users page. In the Active users > Filters search field, type part of the recipient's name, and then press Enter to locate the recipient. If the recipient doesn't exist, then you must create a new mailbox or contact for this user.
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