What the receiver is telling you

This one is about you, not the recipient. The receiver looked at your envelope-from — the bounce address, which your sending tool may have set to something other than your From header — and could not resolve its domain, or resolved it and found no MX. Servers that verify the sender address refuse to accept mail from a domain that cannot receive replies. It happens when a bounce domain was set up without MX, when a tracking or return-path subdomain was never delegated, or when a domain expired under you.

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. Find the actual envelope-from in the bounce or in the sending tool's settings — it is often a subdomain like bounce.yourdomain.com.
2. Make sure that domain resolves and has an MX record. A null MX (a single MX pointing at '.') is also a rejection trigger for sender verification.
3. If the tool uses its own bounce domain, check that it is still configured in the tool; a removed integration leaves a dangling return path.

don't Do not remove the recipient. They did nothing; your return path did.

Decode the whole bounce

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

Common questions