This story was originally published on HackerNoon at:
https://hackernoon.com/how-mailing-lists-break-dmarc-and-why-arc-only-partly-fixes-it.
Why mailing lists break DMARC, what ARC (RFC 8617) actually does about it, why it only helps where the receiver trusts the sealer, and why From-munging is the reliable fix.
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Mailing lists break DMARC because they modify messages and forward from their own servers. ARC lets a list vouch that the message passed DMARC before it touched it, but only helps at receivers that evaluate it and trust the sealer's reputation. From-munging is the universal fix; run both.