How Mailing Lists Break DMARC, and Why ARC Only Partly Fixes It
If you run a mailing list or any kind of forwarder and some of your subscribers quietly stopped receiving mail, DMARC is probably why. This is one of those problems baked into how the standards interact, not a bug you can configure away, and ARC is the patch the industry reached for. It helps less than people expect. Here is the whole picture.
Why lists break DMARC in the first place
DMARC passes when at least one of SPF or DKIM both verifies and aligns with the domain in the visible From header. A mailing list breaks both sides of that.
It resends the message from its own servers, so an SPF check now sees the list's IP against the author's SPF record, which fails. Even if it passed, the list's domain is not the author's, so it would not align.
And it modifies the message. A subject tag like...
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