What the receiver is telling you

Your outbound server tried to find where the recipient's domain accepts mail and could not: no MX record, an MX pointing at a hostname that does not resolve, or an MX pointing back at the sending server itself. The domain may be registered and even have a website — plenty of companies host a site on a domain that has never received mail. For a cold sender it is functionally a dead address.

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.
2. If it is a domain you expected to work, look up its MX; a company that just migrated mail providers sometimes leaves a window with no MX at all.
3. Run domains through an MX check before sending — it is a single DNS query per domain and catches every one of these.

don't Do not retry for days hoping DNS changes. If the company fixes its MX you will find out on the next campaign; retrying a dead domain just adds bounces.

Decode the whole bounce

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

Common questions