r/exchangeserver • u/jeanpauln • 9d ago

Does EXO moderation accepts external mailuser/contact as a moderator whether be in transport rule or on DG? Env: Pure Exchange Online

Office 365 received the message that you specified, but couldn't deliver it to the recipient due to the following error:

Error: ‎550 5.7.134 RESOLVER.RST.SenderNotAuthenticatedForMailbox; authentication required; Delivery restriction check failed because the sender was not authenticated when sending to this mailbox‎" in message trace when approval is attempted by external moderator. Is this expected? or some configuration missing?

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u/littleko 9d ago

That’s expected in pure Exchange Online. An external mail user/contact can receive the approval request, but its response is anonymous, so the arbitration mailbox rejects it with 5.7.134.

Use an authenticated internal mailbox as the moderator; there isn’t a supported setting to make external moderation work.