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"550 5.1.11 RESOLVER.ADR.ExRecipNotFound" when delegate sends update to meeting after manager moved to Microsoft 365 hybrid environment


Consider the following scenario:

  • A manager, who has a delegate, has their mailbox moved to Microsoft 365.
  • The delegate's mailbox remains in on-premises environment.
  • The manager creates a meeting on their calendar after the mailbox is migrated.
  • The delegate updates the meeting time and sends an updated version of the meeting to the invitees.

In this scenario, the delegate receives a non-delivery report that includes the following IMCEAEX address and error message:

Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup'


The legacyExchangeDN attribute value of the invitee's cloud object is stored in the calendar item. If the Microsoft Entra Connect (Microsoft Entra Connect) Exchange hybrid deployment settings aren't enabled, the legacyExchangeDN attribute values in the cloud aren't written back to the on-premises.


To resolve this issue, enable the Exchange hybrid deployment settings in Microsoft Entra Connect. For more information, see Exchange hybrid writeback.