The Simple Mail Transfer Protocol (SMTP) is the standard method for sending emails across the internet. When an email is sent or received using SMTP, status codes are returned that indicate whether the operation succeeded or failed in some way. Understanding SMTP status codes can help diagnose issues and improve deliverability.
These 3-digit SMTP status codes are defined in RFC 5321. The first digit indicates a broad category:
- 2xx: Success
- 4xx: Temporary failure
- 5xx: Permanent failure
The next two digits provide a more specific status. Some common SMTP status codes include:
A 250 status indicates the requested email action was completed successfully.
For example, 250 OK may be returned when:
- An email is submitted for delivery by the sending mail server
- The recipient mail server accepts the message
- A recipient mailbox is available and ready to receive emails
250 statuses mean everything worked as expected, and no further action is required. Emails can now be safely delivered to recipients.
550 Not Found
A 550 SMTP status means the recipient’s mailbox was not found or does not exist.
Some reasons for 550 Not Found errors:
- The recipient address is invalid or does not exist on the destination mail server
- The recipient’s account has been disabled or deleted
- There is a configuration issue with the recipient mail server
550 statuses indicate delivery failure. The sending mail server may retry delivery temporarily before eventually bouncing emails back to senders with this permanent error.
451 Temporary Security Failure
The 451 status indicates a temporary security issue prevented email delivery.
- Greylisting is blocking message delivery until it passes spam checks
- DMARC or spam filter checks flagged the message as unsafe
- The recipient mail server requires encrypted TLS connections
With a 451 status, the sending mail server will typically retry delivery later when the security restriction or error risk has cleared.
421 Server is Too Busy
A 421 status code means the recipient mail server or host is overloaded and unable to accept more connections right now.
This is a temporary state. The sending mail server will queue the messages and retry delivering them after a delay.
Servers may return 421 errors during peak traffic spikes or when technical issues arise like:
- Running out of disk space, memory, or CPU capacity
- Crashing due to a software bug or hardware failure
421 statuses indicate a transient issue, so emails are not lost. They will send once the destination mail server recovers.
- SMTP status codes indicate success or failure when sending or receiving emails
- 250 OK means the email action completed successfully
- 550 Not Found errors mean the recipient mailbox does not exist
- 451 Temporary Security Failure means greylisting or spam filtering is blocking delivery temporarily
- 421 Server Too Busy means the destination cannot accept more messages right now
SMTP status codes provide useful diagnostic information about email delivery issues. While some statuses like 550 show permanent failures, many indicate temporary problems.
Understanding the meaning behind status codes like 250, 451, and 421 lets mail server operators and email administrators better troubleshoot issues and improve deliverability.
With appropriate handling of each status code, companies can ensure successful email communication and avoid losing messages.
Frequently Asked Questions about SMTP Status Codes
What is an SMTP 250 status code?
250 is an SMTP status code indicating an email action was completed successfully. For example, the receiving server accepted the message or the recipient mailbox exists and is ready.
What does SMTP 550 error mean?
550 signifies a permanent failure because the intended recipient address does not exist or their mailbox is unavailable. The sending mail server should not retry delivery.
Why does my mail server return 451 SMTP status?
A 451 error means a temporary security issues is blocking delivery, such as greylisting spam checks or DMARC authentication failures. The sending mail server should retry sending later.
What is SMTP status 421 and what causes it?
421 means the receiving mail server or host is overloaded so it is refusing connections temporarily. The 421 status indicates a transient issue that the sending server should retry after.
How long do I wait to retry after SMTP 451 status?
It’s recommended to retry delivering an email every 15 minutes when you receive a 451 security error, for up to 4 hours. This gives time for temporary spam checks to clear while still delivering promptly.
Should I be concerned about 250 SMTP warnings?
No, 250 SMTP status codes indicate successful operations, not warnings. As long as email servers return 250 OK in response to sending messages, everything is working correctly.
What is the SMTP status code for invalid email address?
If an email address does not exist on the receiving mail server, it will return a permanent 550 error indicating the mailbox is unavailable or not found.
Why am I getting relay access denied SMTP error?
A 554 SMTP error with “relay access denied” means the receiving mail server has rejected the message to prevent unauthorized relaying. The sender’s mail server must authenticate before sending messages through another server.
What does 421 too many connections mean?
A 421 error with “too many connections” indicates the receiving mail server has reached its concurrent connection limit. This is usually a temporary issue that the sending mail server should retry after a delay.
How long will mail server retry delivery after error?
Most mail servers will retry message delivery for up to 4-5 days on temporary errors before bouncing the email back to senders with permanent failure notifications.
Should I retry delivery after unknown SMTP host?
No, receiving an unknown host error means the domain name does not resolve to a valid mail server IP address. This is a permanent error and the message will never be deliverable.
What is SMTP code for mail server not responding?
If the destination mail server is unreachable and not responding to connections, the sending server will usually return a general connection timeout or network error status code.
What does SMTP status 503 mean?
A 503 error indicates an unexpected DNS failure or mail server outage rather than an individual email delivery failure. This usually points to a wider technical issue that requires resolution.
Is SMTP status 554 permanent failure?
Yes, SMTP status codes that start with 5 signify permanent failures, so the sending server should not retry delivery after getting an error starting with 54 or 59.
What should I do if recipient never receives email?
First check for typos or spelling errors in the email address, then review SMTP logs on the sending & receiving servers to identify any bounce messages or errors related to that recipient.
How long do mail servers keep SMTP logs?
Most mail servers only retain SMTP log history for a few days or weeks. However, some servers can be configured to archive logs indefinitely for auditing or debugging purposes.
How can I decode SMTP status codes?
The first SMTP status digit indicates a broad category, where 2 means success, 4 is temporary failure, and 5 means permanent error. The next two digits provide more detailed diagnostic codes.
Where can I look up SMTP status codes?
SMTP statuses are defined in RFC 5321, but you can also find common codes listed on various tech sites. Major email services may also provide explanations of statuses you receive.