About the Bounce Explainer
Bounce messages contain a three-digit SMTP code and often an enhanced status code such as 5.1.1. Codes starting with 5 are permanent failures (hard bounces); codes starting with 4 are temporary (soft bounces) and the sender retries automatically.
The explainer recognises the codes and wording used by Gmail, Microsoft, Yahoo and common blocklists, and tells you whether the problem is the address, your reputation, your authentication or the receiving server.
Hard vs soft bounces
5xx codes are permanent failures (hard bounces): the sender will not retry. 4xx codes are temporary (soft bounces): the sender retries automatically. The three-number code like 5.7.26 tells you the exact category.