ARC: The "Trust Me Bro" of Email Authentication

There’s a lot of noise in the email world lately about ARC (Authenticated Received Chain). People talk about it like it’s the missing puzzle piece in email authentication. But after watching it float around in production for years, I’ve come to a pretty simple conclusion:

ARC is basically just a “trust me bro” header.

Let’s back up for a second. What’s ARC supposed to do?

ARC tries to fix the problem where SPF and DKIM fail because an email got forwarded. Imagine a university forwards an email to your Gmail account, and suddenly Gmail sees the wrong IP or modified headers. DMARC fails. ARC steps in and says, “Hey, don’t worry. The person before me checked SPF, DKIM, and DMARC. Here’s what they saw. Trust me, they did their homework.”

It’s a cool idea on paper. But here’s the problem.

Trust isn’t transitive

Just because someone says they trusted it doesn’t mean you should. If I forward you an email and tell you it’s legit, that only matters if you already trust me. ARC doesn’t change that. It just wraps that idea in a signed header.

And who are we supposed to trust? Should Gmail rely on ARC results from a random guy running a mail server on a dusty old VPS? Should I trust ARC results from someone who forgot they set up DKIM back in 2019? There’s no universal trust network here. Everyone is just guessing who’s credible.

What actually happens in the real world?

I haven’t seen a single provider say, “We now let this email through because of ARC.” Not once. Not even Google, who helped design it.

We all keep forwarding email the same way we always have. If forwarding breaks authentication, it still breaks it. ARC just adds headers. That’s it. The only places using ARC in any meaningful way are already putting effort into making forwarding work, and even they treat ARC as optional metadata. Nothing more.

Yes, spammers can fake it

There’s nothing stopping a spammer from signing their own ARC headers. “Hey, I totally checked SPF and DKIM and it passed. Look, I signed it myself.” Cool story. I’ll file that next to the Bitcoin giveaway.

Receivers can choose which ARC signers to trust. But that puts us right back where we started. Trust is still subjective. And most of us have no reason to build a hand-curated list of who we trust to sign ARC headers.

Bottom line?

ARC is a clever technical idea. I’m not calling it useless. It has potential. But in practice, it’s not solving a real problem for most providers. It’s not improving deliverability. It’s not making forwarding more reliable in any noticeable way. And I’ve never once heard someone say ARC was the reason they trusted a message.

So for now, we’re not doing anything with ARC at MXroute. If that ever changes, it’ll be because it makes things measurably better for real people.

Until then, ARC is just another header.

Trust me bro.

- Jarland