emailrules.today
Stop 6 of 8Week oneInstant, and invisible

Hard bounce

A permanent failure — the address does not exist or will never accept mail. Stop sending.

Say it out loud

“That address does not exist. We stop sending to it immediately and permanently.”

A hard bounce is the receiving server saying the failure is permanent: unknown user, dead domain, blocked outright. Mainstream platforms suppress these automatically. Continuing to mail them is one of the clearest signals of a list nobody is maintaining.

Email headerThe number tells you which kind it isPublished valuesscrolls →
550 5.1.1 The email account that you tried to reach
does not exist.

5xx = permanent. That is the entire rule: read the first digit.

550 5.7.1 Message rejected due to policy
553 5.1.2 Domain name not found
vs a soft bounce:
452 4.2.2 Mailbox is over quota

4xx = temporary. The server is asking you to come back later, not telling you to stop.

First digit 5, stop forever. First digit 4, try again. Everything else in bounce handling is detail.

Your platform

Klaviyo, Mailchimp and friends do this automatically.

What goes wrong

Re-importing an old list that reintroduces addresses your platform already suppressed. Suppression lists exist for a reason and a CSV upload is how they get overruled.

The dated rule behind this

A definition is not a citation. These are the pages with the primary source, the date it was published, and what to do about it.

See also

Where this sits

Stop 6, the verdict lands. The message goes to an inbox, a spam folder, a refusal, or an address that was never a person.

← BeforeWarm-upNext →Soft bounce

FAQ

Awkward questions

Who is this actually for?

People who ship email and are too busy to re-read every PDF: week-one marketers, lifecycle/CRM, deliverability, multi-country ops, DTC brands, agencies — on Klaviyo, Mailchimp, Braze, HubSpot, SFMC, or something else. Pick your tool and geos so product-specific pages only appear when they match. EU and UK are first-class filters. Not for people who sell tools about email and need a fake score to demo.

Why only ~40 rules? Isn’t that… thin?

On purpose. A wall of 400 undated “best practices” is how you look busy and still get burned. Every page needs a primary source we actually read. Thin and checkable beats fat and folklore. Europe, bulk inbox rules, measurement honesty, and protocol topics like BIMI/DMARC policy are on the shelf when we can cite them — not every Member State and not every vendor myth. Coverage map lists what we still refuse to invent.

Is this legal advice?

No. If it were, you’d have a billable hour and a longer PDF. This is a dated operator reference written by an email geek. Confirm anything high-stakes with counsel who knows your facts. Same for “will this make me compliant?” — no tool does that. Lawyers and judges do. We say what the sources say and what to do first on Monday.

Why no trust score out of 100?

Because we refuse to invent a number you can’t audit. Fake scores sell seed tests and panic. You get findings, dates, and links. If that feels less exciting than a red dial, good — you’re not the red-dial customer.

Why should I trust you more than my ESP’s blog?

ESPs sell seats. Seed-score vendors sell fear. AI invents citations when nobody watches. We sell nothing today — no pixels, no placement scores, no affiliate — so we can say when a tool is the problem. A human verifies, dates, and corrects in public. Tools may help draft; they do not ship unsourced claims. Check the primary links. Fail that test and leave.

Do I need an account?

No. Filters live in this browser and the URL. Share the link. That’s it. Accounts come later only if they earn it — not so we can nurture you about email.

I’m an agency. Where’s multi-client mode?

We tried a client-name CRM on the setup card. It made the product feel like work before it felt useful. Role filters + copy link + one-page brief (optional PDF title) is enough for now. Complexity comes back when the free shelf is habit, not before.

Is the quiet changelog a bug?

No. Quiet means nothing material moved. We don’t invent urgency so the homepage looks “alive.” Sticky risks still show what usually needs a person when the market is still.