Trusted ARC sealer
An intermediary whose ARC results a receiver has chosen to believe, so mail it forwards can be accepted even when SPF, DKIM or DMARC now fail.
What it means
ARC only helps if the receiver trusts the server that sealed the chain, since anyone can add ARC headers. Microsoft 365 lets administrators add trusted ARC sealers by domain, for intermediary services whose changes to messages (to headers, content or attachments) make authentication fail on the way into Exchange Online.
Microsoft advises adding only legitimate services that are actually needed, since a trusted sealer can make failing mail pass.
If messages that pass through a filtering service land in Junk or quarantine for failed authentication, the service's sealing domain belongs on the list.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Microsoft Learn: Configure trusted ARC sealerslearn.microsoft.com
- RFC 8617: The Authenticated Received Chain (ARC) Protocol, section 5rfc-editor.org
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