What the receiver is telling you

The address itself is malformed — a space inside it, two @ signs, a missing domain, a stray quote or bracket from a spreadsheet export. The receiver rejected it before looking anything up. This is a list-hygiene failure, not a deliverability one, but the bounce still counts as a hard failure on the sending domain's record.

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. Open the address and look at it. The fault is usually visible: trailing punctuation, a name pasted into the email column, a mailto: prefix.
2. Fix the export. Syntax errors arrive in batches from the same CSV, so the rest of that file has more.
3. Run a syntax check before the list reaches the sending tool; every list cleaner does this in milliseconds.

don't Do not let the sending tool retry it. A malformed address is malformed forever.

Decode the whole bounce

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

Common questions