SMTP error codes: What they mean and how to fix them

SMTP error codes overview:

• SMTP error codes are three-digit reply codes that SMTP servers return when they defer or reject an email, showing whether delivery failed temporarily (4xx) or permanently (5xx).
• Understanding SMTP error codes helps you troubleshoot bounces faster by separating “retry later” issues (like busy servers or full mailboxes) from “fix first” issues (like invalid addresses or policy blocks).
• Enhanced status codes (such as 5.1.1 or 5.7.1) add extra detail, making it easier to identify the likely cause and next step.

SMTP servers return short, three-digit reply codes when they defer or reject an email. These SMTP error codes indicate whether delivery encountered a temporary issue (4xx) or failed permanently (5xx), often due to mailbox storage limits or domain authentication issues.

Use Sendmarc’s domain checker to confirm that your DNS records are correctly configured and to identify authentication issues that may be behind SMTP error codes.

Explaining SMTP error codes

During an SMTP conversation, the sending server issues commands, and the receiving server answers with responses. Each server response usually contains:

1. A three-digit SMTP status code, such as 421, 450, or 550
2. An optional enhanced status code, such as 4.2.2 or 5.7.1
3. Sometimes a short, human-readable message, such as “Authentication required”

First digit: Temporary vs. permanent

The first digit of an SMTP error code tells you the status code:

• Code 4xx – Persistent Transient Failure. The request failed for now, but it may succeed later.
• Code 5xx – Permanent Failure. The request won’t succeed until something changes.

When you interpret SMTP error codes, it helps to focus on the difference between 4xx and 5xx:

4xx SMTP error codes mean temporary issues:

• Mailboxes may be full, or servers might not be available.
• Email systems usually retry delivery after a 4xx response.

5xx SMTP error codes mean permanent issues:

• Addresses can be invalid, domains misconfigured, or syntax incorrect.
• Email systems mark 5xx responses as hard bounces.

Enhanced SMTP status codes

Many systems add Enhanced Status Codes to SMTP error codes to describe the problem in more detail. These typically have three parts, such as 5.1.1 or 4.7.1, and they include:

Class:

• Code 4.x.x – Persistent Transient Failure.
• Code 5.x.x – Permanent Failure.

Subject:

• 0.XXX – Other or undefined status
• 1.XXX – Addressing status
• 2.XXX – Mailbox status
• 3.XXX – Mail system status
• 4.XXX – Network and routing status
• 5.XXX – Mail delivery protocol status
• 6.XXX – Message content or media status
• 7.XXX – Security or policy status

A few common examples:

• Code 4.2.2 often maps to a mailbox that’s full
• Code 5.1.1 indicates an invalid or unknown address
• Code 5.7.1 signals a security or policy failure

Reading the basic SMTP error code alongside the enhanced status code provides a clear picture of what went wrong and who likely needs to act.

Connect enhanced codes with authentication

Codes in the X.7.XXX family, such as 5.7.0 or 5.7.1, indicate that a security or policy rule or requirement prevented the message from being accepted.

Use Sendmarc’s free domain checker to confirm that:

• Your Sender Policy Framework (SPF) records include all authorized senders.
• Your DomainKeys Identified Mail (DKIM) keys are published correctly.
• Your Domain-based Message Authentication, Reporting, and Conformance (DMARC) record is valid.

This helps you quickly see whether misconfigured authentication is contributing to the SMTP error codes you’re seeing.