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Consider the following scenario:
In this scenario, the moderator receives a non-delivery report (NDR) that states the following:
This issue occurs because the email message for moderation is sent from an arbitration mailbox in on-premises, but arbitration mailboxes are excluded from Azure AD Connect. They aren’t present in Exchange Online as recipients.
To fix this problem, make sure that the distribution group and moderator’s mailbox are located in the same place. For example, if the distribution group is located in Exchange Online, the mailbox of the moderator should also be located in Exchange Online.
Keywords: Someone tried sending an email message to my organization and received a non-delivery report (NDR), kbContentAuto, kbSupportTopic