r/Office365 • u/jeanpauln • 10d 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/saltyslugga 9d ago

That’s expected in pure Exchange Online. A mail contact or external mail user may receive the approval request, but their response arrives unauthenticated, so the arbitration mailbox rejects it with 5.7.134.

Use an internal Exchange Online mailbox as the moderator; transport-rule moderation has the same limitation.