SMTP codes indicate whether an email was processed successfully or why a step in the delivery process failed.

They help narrow down delivery problems. It is particularly important to consider not only the digits of the SMTP code, but also the text message from the responding server.

Where can I find SMTP codes?

You can find SMTP codes in Email Live Tracking.

1. Sign in to the Control Panel using your administrative credentials.
2. Navigate to Email Security > Email Live Tracking.
3. Search for the affected email.
4. Open the email's context menu.
5. Click Email details.
6. Open the SMTP tab.
7. Check the displayed SMTP code and the associated server response.

Advanced email information

Structure of an SMTP code

An SMTP code consists of three digits, for example:

550

The first digit indicates the general evaluation of the response:

2xx codes

A 2xx code means that an SMTP step was successful.

Example:

250 OK

This means that the receiving server accepted the request.

3xx codes

A 3xx code indicates that an intermediate step in the SMTP dialog is required.

These codes normally occur during communication between mail servers and are generally less relevant for troubleshooting than 4xx or 5xx codes.

4xx codes

A 4xx code indicates a temporary error.

This does not automatically mean that the email was permanently rejected. Hornetsecurity will attempt delivery again later.

Possible causes include:

• The destination server is temporarily unreachable.
• The destination server is not currently accepting connections.
• The mailbox or server is temporarily unavailable.
• Greylisting is active.
• The destination server reports a temporary restriction.

Example:

421 Service not available

In this case, check whether the destination server is reachable. If the email has the status Delayed, Hornetsecurity will make additional delivery attempts.

5xx codes

A 5xx code generally indicates a permanent error.

The email was not delivered. The cause must be investigated and resolved before sending again can be successful.

Possible causes include:

• The recipient address does not exist.
• The destination server rejects the email.
• The mailbox is permanently unreachable.
• The email was rejected due to a policy.
• Authentication or relay permission is missing.

Example:

550 5.1.1 User unknown

In this case, check whether the recipient address is correct and whether the mailbox exists.

Extended SMTP codes

In addition to the three-digit SMTP code, an extended status code may be displayed, for example:

550 5.5.4 Invalid domain name

In this example:

• 550 is the SMTP code
• 5.5.4 is the extended SMTP code
• Invalid domain name is the server response

The extended SMTP code narrows down the error more precisely. The server's text message should always be checked as well, since it often states the specific reason.

Notes

Observe the following points:

• The text message next to the SMTP code is just as important as the code itself.
• A 4xx code does not automatically mean that the email was permanently rejected.
• A 5xx code generally means that delivery has permanently failed.
• Multiple delivery attempts with different SMTP responses may be visible in Email Live Tracking.

Additional information

For more information, see the following articles and technical references:

• Why is my email not being delivered?
• Automatic email delivery when the mail server is temporarily unreachable
• Manual article: Categorization reasons