SMTP bounce codes follow a predictable pattern. Once you read the pattern, every bounce becomes diagnosable.
5xx codes. Permanent failures. 550 is the workhorse: "mailbox unavailable." Suppress immediately.
4xx codes. Transient failures. 421 is "service not available, try later" — usually rate limiting. 451 is "local error, try later" — server overload or temp policy.
The enhanced status code. Modern mailbox providers include a three-part code like 5.1.1 (bad destination address) or 5.7.1 (delivery not authorized — typically reputation). Find the enhanced code in the full SMTP response.
Provider-specific codes. Major providers attach context strings. Google's 550-5.7.1 "Our system has detected..." is reputation; suppress and investigate. Yahoo's 421 4.7.0 [TS03] is throttling; back off.
Bounce categorization. Inbox OSS classifies every bounce into hard, soft, OOO, content-block, policy-block, and reputation-block. The classification drives auto-suppression policy.
The diagnostic loop. When a bounce class spikes, look at the full SMTP response. The provider almost always tells you exactly what is wrong if you read carefully.