On behalf of reply from a resource delegate to the meeting organizer in Microsoft 365

Original KB number:   2879458


In Microsoft 365, assume that one or more delegates are added to a resource mailbox, and that a delegate accepts a meeting request. In this situation, the From line of the email message that's sent to the meeting organizer says <Delegate> on behalf of <ResourceMailbox>.


When a delegate is added to a resource mailbox through the Select delegates who can accept or decline booking requests option, the delegate is granted Send on Behalf permissions. This behavior is by design in Microsoft 365.


Go to the Exchange admin center, and then open the settings for the resource mailbox. In the navigation pane, select mailbox delegation, and then under Send As, add the delegate.


Don't remove the delegate who is listed under Send on Behalf. Removing the delegate also removes delegate access.

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