SMTP status codes explained: 2xx, 4xx, 5xx and enhanced codes
An SMTP error often contains two related codes: a three-digit reply such as 550, and an enhanced status such as 5.1.1. The first says whether delivery succeeded or can be retried; the second narrows down what failed.
Updated July 15, 2026
Read the first digit first
| Class | Meaning | Retry? |
|---|---|---|
| 2xx | The requested SMTP action succeeded | No retry needed |
| 4xx | Temporary failure or deferral | Yes, with backoff |
| 5xx | Permanent failure | Not until the cause changes |
A message may encounter several SMTP replies while it moves through a transaction. The final recipient-specific result in a delivery-status notification is the one that tells you what happened to that recipient.
How X.Y.Z enhanced codes work
In X.Y.Z, X is success, temporary failure, or permanent failure. Y is the subject: address, mailbox, mail system, network/routing, protocol, content/media, or security/policy. Z supplies more detail.
| Code | Typical meaning |
|---|---|
| 5.1.1 | Bad destination mailbox address |
| 4.2.2 | Mailbox full or over quota temporarily |
| 5.3.4 | Message too large |
| 4.4.1 | No answer from destination host |
| 5.7.1 | Delivery not authorized; surrounding text explains the policy |
Why provider text still matters
A standard code deliberately covers a family of failures. For example, 5.7.1 can accompany relay denial, an authentication rule, reputation filtering, or another local policy. Read the sentence, URL, and provider-specific identifier beside it before choosing a fix.
Decode a real error
Diagnostic-Code: smtp; 550 5.1.1 The email account does not existPaste your complete error into the bounce analyzer to separate standardized fields from provider prose and get safe retry guidance.