SMTP & bounce code lookup

A 4xx code is transient — a temporary failure the sending server will retry (rate limiting, greylisting, a busy server, a full mailbox). A 5xx code is permanent — the message was rejected and won't be retried (bad address, blocked by policy, authentication failure). 4xx maps to a soft bounce, 5xx to a hard bounce.

It's a permanent rejection on policy/security grounds — the receiver refused the message. Common causes are poor sending reputation, content filtering, or failed SPF/DKIM/DMARC alignment. It is not a bad-mailbox error; the fix is to check your authentication and reputation, not to remove the address.

It's the dotted code (like 5.7.1) defined by RFC 3463. The first digit is the class (2 success, 4 transient, 5 permanent), the second is the subject (addressing, mailbox, network, security…), and the third is the specific detail. It gives a more precise reason than the three-digit basic code alone.

Not on the first soft (4xx) bounce — those are temporary and retried. But repeated soft bounces over time should be suppressed, and any hard (5xx) bounce for a bad mailbox (5.1.1) should be removed immediately. High bounce rates hurt reputation, so good list hygiene matters either way.