Originally posted by Microsoft
Jun 17, 2022
Exchange Online
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Authenticated Received Chain (ARC) is an email authentication mechanism that helps preserve authentication results across intermediaries. Email authentication mechanisms like SPF, DKIM, DMARC are used to verify the senders of emails for the safety of mail recipients, but some legitimate services may make changes to the email between the send and receipt. This intervention from legitimate services may accidentally cause the message to fail email authentication at subsequent hops.
The ARC trusted sealers feature lets admins add trusted intermediaries in the Microsoft 365 Defender portal. This allows Microsoft to honor ARC signatures from your list of trusted intermediaries, to help authenticate the message.

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