What the receiver is telling you

The receiving server looked up the local part — everything before the @ — in its directory and found nothing. The domain is real and accepts mail; the person is not there. In a cold email list this is a typo, a scraped address that was guessed from a name pattern, or someone who left the company. Google and Microsoft count every one of these against you: a sender whose mail is addressed to people who do not exist is, by definition, not sending to people who asked for it.

What a hard bounce means for your reputation

Receivers count hard bounces. Gmail and Microsoft both treat a sender whose mail is addressed to people who do not exist as a sender who did not get those addresses from the people themselves, which is the definition of unsolicited. The working threshold is two percent of a send: under that is noise, above it starts to cost domain reputation, and above five percent a fresh domain can be throttled after a single campaign. The fix is upstream — verify before sending — not downstream.

What to do

1. Remove the address from every list and sequence, not just the current one.
2. If more than two percent of a send came back 5.1.1, stop and verify the rest of the list before the next campaign — the ones that bounced are a sample of the ones about to.
3. Check whether the addresses were generated from a name pattern (first.last@) rather than verified; pattern-guessed lists run ten to thirty percent 5.1.1.
4. If it is a domain you know is valid and the person is definitely there, check for a typo in the local part and a stray character from the export.

don't Do not retry. The mailbox will not appear tomorrow, and each retry is another bad address on your record.

Decode the whole bounce

Paste the complete failure notice — every code and the receiver's wording — and get each part explained together.

Common questions