SMTP Response Codes: Complete Reference for Troubleshooting Email Delivery

Every SMTP command receives a three-digit numeric response code. These codes are the protocol’s way of communicating success, failure, or temporary conditions between servers. Understanding them is essential for troubleshooting delivery issues.

Response Code Structure

SMTP response codes follow a consistent format:

ddd[ -]text

The first digit indicates the class (success, information, etc.).
The second digit indicates the category (syntax, connection, etc.).
The third digit provides further specificity within the category.
A dash (-) after the code means more lines follow; a space means this is the final line.

First Digit (Class)

Class	Meaning	Behaviour
2xx	Success	The command was accepted and completed.
3xx	Intermediate	More information is needed (e.g., DATA content).
4xx	Temporary Failure	Try again later — the condition may resolve.
5xx	Permanent Failure	The command was rejected — do not retry.

Second Digit (Category)

Digit	Category
x0x	Syntax / system
x1x	Information / help
x2x	Connection / transmission
x3x	Authentication / security
x5x	Mail system / recipient status

Common Response Codes

2xx — Success

Code	Meaning	Context
211	System status help	Response to `HELP`.
220	Service Ready	The server is ready. Sent at connection start.
235	Authentication Succeeded	The `AUTH` command completed successfully.
250	Request Completed	The command (e.g., `MAIL FROM`, `RCPT TO`) succeeded.
251	User Not Local	The recipient is not local, but the server will forward the message.
252	Cannot Verify User	The server cannot validate the recipient but will attempt delivery.

3xx — Intermediate

Code	Meaning	Context
334	Server Challenge	The server is challenging the client during SASL authentication.
354	Start Mail Input	The server is ready to receive the message body after `DATA`.

4xx — Temporary Failure (Retry Later)

When a server returns a 4xx code, the sending MTA should queue the message and retry. If delivery still fails after a configured retry period (typically 2–5 days), the message is bounced.

Code	Meaning	Common Cause
421	Service Not Available	The server is shutting down, overloaded, or temporarily unavailable.
450	Mailbox Unavailable	The recipient’s mailbox is temporarily locked or busy.
451	Local Error	The server encountered a temporary processing error.
452	Insufficient Storage	The server’s disk or mail store is full.
453	TLS Not Available	The server cannot negotiate TLS as required.

5xx — Permanent Failure (Do Not Retry)

A 5xx response means the command was rejected. The sending MTA should not retry — it should generate a bounce immediately.

Code	Meaning	Common Cause
500	Syntax Error	The command was not recognised or was malformed.
501	Syntax Error in Parameters	The arguments to the command were invalid.
502	Command Not Implemented	The server does not support this command.
503	Bad Sequence of Commands	The client issued commands in the wrong order (e.g., `DATA` before `RCPT TO`).
504	Parameter Not Implemented	The server supports the command but not this particular parameter.
521	Server Does Not Accept Mail	The host does not accept mail at all (e.g., a firewall or relay blocker).
523	Encrypted Connection Required	The configured policy requires TLS but the connection is unencrypted.
530	Authentication Required	The server requires authentication before accepting mail.
535	Authentication Failed	The provided credentials were rejected.
550	Mailbox Not Found	The recipient address does not exist. Hard failure.
551	User Not Local	The server is not the final destination — try a different server.
552	Mailbox Full / Exceeded Quota	The recipient’s mailbox has exceeded its storage limit.
553	Mailbox Name Not Allowed	The recipient address format is invalid or the domain is not accepted.
554	Transaction Failed	Generic failure — the message was rejected (often used by spam filters).

Enhanced Status Codes (RFC 5248 / RFC 2034)

In addition to the three-digit numeric code, SMTP servers may include an enhanced status code — a structured, dot-delimited code that provides finer granularity:

250 2.1.5 Recipient OK
550 5.1.1 The email account does not exist

The enhanced code format is c.x.y:

Component	Meaning	Example
c	Class (same as above)	2 = success, 4 = temp fail, 5 = perm fail
x	Subject	1 = addressing, 2 = mailbox, 3 = mail system, 5 = security
y	Detail	Further precision within the subject

Common Enhanced Codes

Enhanced Code	Meaning
2.1.5	Recipient address accepted.
4.2.1	The mailbox is temporarily unavailable.
4.4.1	Unable to connect to the destination MTA.
4.7.1	Delivery not authorised — policy or security failure.
5.1.0	Other address status — generic addressing failure.
5.1.1	Mailbox does not exist (hard bounce).
5.1.2	Bad syntax in the recipient address.
5.2.2	Mailbox is full.
5.7.1	Delivery not authorised (e.g., SPF failure).
5.7.26	The message requires TLS but the connection was not encrypted.

Error Handling Patterns

Retry Logic for 4xx

When your MTA receives a 4xx response:

Queue the message with a retry timestamp.
Retry with exponential backoff — typical starting intervals are 5–15 minutes, doubling up to a maximum of 4–6 hours.
Set a maximum retry window — commonly 2–5 days.
Bounce on expiry — if the message is still undelivered after the retry window, generate a permanent failure notification to the sender.

Hard Bounce Handling for 5xx

When your MTA receives a 5xx response (especially 550 5.1.1):

Do not retry. The address is invalid.
Generate a bounce to the Return-Path address immediately.
Suppress the address in future mailings if the bounce indicates a non-existent mailbox.
Log the failure with sufficient context for postmaster review.

Multi-Line Responses

Some SMTP responses span multiple lines. The server signals continuation with a dash after the code, and terminates with a space:

250-SIZE 35882577
250-8BITMIME
250-PIPELINING
250 AUTH LOGIN PLAIN

Each line after the first uses code- text, and the final line uses code text. This is standard in EHLO responses listing supported extensions.

Quick Reference Table

Code	Meaning	Action
220	Service Ready	Connection accepted.
235	Auth Success	Proceed with `MAIL FROM`.
250	OK	Command succeeded.
354	Start Input	Send message body.
421	Not Available	Retry later (server issue).
450	Mailbox Busy	Retry later (recipient issue).
452	Storage Full	Retry later (disk issue).
550	No Such User	Bounce immediately.
552	Mailbox Full	Bounce (inform sender).
554	Rejected	Check spam / policy rules.

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