What the receiver is telling you

A permanent refusal. By itself 550 says only 'no, and do not retry'. It covers two completely different families of failure: the address is wrong (5.1.x, 5.2.x, 5.4.1) and the sender is refused (5.7.x). Sending tools lump both under 'bounced', which is how good addresses get scrubbed after a reputation block and dead addresses get retried after a policy fix. Always read the enhanced code.

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. Look at the dotted code that follows. 5.1.x, 5.2.1, 5.4.1: bad address — remove it. 5.7.x: policy — keep the address, fix the sending side.
2. If there is no enhanced code, read the text: 'user unknown', 'does not exist', 'no such user' are address problems; 'blocked', 'rejected', 'policy', 'spam', 'reputation' are yours.
3. Paste the whole line into the bounce decoder if it is ambiguous.

don't Do not treat every 550 as a bad address. Roughly half of the 550s a cold email fleet sees are policy blocks where the address was fine.

Decode the whole bounce

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

Common questions