5.1.1 vs 5.7.1: what your bounces are actually telling you

Two campaigns both show a five percent bounce rate. One has a data problem you can fix in an afternoon. The other has a reputation problem that will take weeks to repair and is quietly getting worse. The dashboard number is identical. The only way to tell them apart is to read the code inside the bounce — the DSN status — and most tools never show it to you.

The two that matter most

5.1.1 means the address does not exist. The mailbox is not there. This is a data problem: your list is stale or was never verified. The fix is to clean the list and stop mailing dead addresses — because every one of them also damages your reputation with the provider.

5.7.1 means you were blocked for policy or reputation. The address is probably fine; the receiver decided not to accept mail from you. This is a sending problem: volume too high, content tripping filters, or a domain whose reputation has slipped. Cleaning your list will not touch it. Slowing down and fixing authentication might.

The rest of the map

• 5.2.2 — mailbox full. Temporary, on their side. Harmless in small numbers.
• 5.4.x — routing or DNS trouble reaching the domain. Often the domain, not you.
• 4.x.x — not a bounce at all but a temporary defer; the message may still deliver on retry.

The leading digit tells you permanence — 5 is permanent, 4 is temporary. The middle and last digits tell you the reason. Together they turn “it bounced” into “here is exactly what to do next.”

A quick diagnostic

1. Split your bounces by DSN code, not just a total. A single rate is uninformative.
2. If 5.1.1 dominates: your problem is data. Verify the list, drop the dead addresses, and do not resend to them.
3. If 5.7.1 dominates: your problem is reputation. Cut volume, check SPF, DKIM and DMARC, and slow the inbox down.
4. If it is a mix, fix the data first — bad addresses feed the reputation problem, so cleaning the list helps both.

CraftNudge reads the DSN code out of every bounce and files it by cause, so the answer to “why did these bounce” is on the screen instead of in your inbox, unparsed.