My Waitlist Emails Passed SPF and DKIM but Failed DMARC

New here? I'm building Vesprit in public for Shipaton, a study app that predicts what you're about to forget and brings it back before it's gone. I wrote a full intro to it here. This post is one bug from that build: the week my waitlist quietly broke, and why "my domain is authenticated" turned out to mean nothing.

The Setup

Vesprit has a waitlist. Someone enters their email, the form posts to my email provider, they get a confirmation email, they click it, they're on the list.

Every part of that worked. The form returned success. The subscriber showed up stored. The confirmation email sent.

And landed in spam. Every time. Under double opt-in that isn't cosmetic: an unconfirmed subscriber never gets anything again, so a signup in spam is a signup I don't have, and nothing anywhere reports it as a failure.

So I did what the...

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