What is the difference between a 2xx, 4xx, and 5xx SMTP response?
- 2xx codes mean success.
- 4xx codes mean Transient Error (a temporary failure).
- 5xx codes mean Permerror (a permanent failure).
If we compare it to shipping signals: 2xx means "cleared to dock," 4xx means "maybe try again later," and 5xx means "entry denied."
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