SMTP Error 451:

What does it mean? And How to resolve this error.

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Error Description

SMTP error 451 is a transient error message that is returned by a mail server when it is temporarily unable to process an incoming email message. It is often accompanied by a specific error message or code that provides more information about the reason for the delay. This error code falls under the category of "4xx" errors, which are temporary errors that suggest a problem that may be resolved in the future.

What's Causing This Error?

SMTP error 451 can be caused by various factors, including:

  • Server congestion or overloading: When a mail server is processing a high volume of incoming email traffic, it may temporarily reject new connections or delay email delivery to reduce its workload.
  • Greylisting: Some mail servers use greylisting as a spam-fighting technique. When an unknown sender attempts to send an email, the receiving server may initially reject it with a 451 error. If it's a legitimate sender, the sending server will retry the delivery after a delay, and the email will be accepted.
  • Rate limiting: Mail servers may impose limits on the number of emails a sender can send within a specific time frame. If this limit is exceeded, the server may respond with a 451 error.
  • Temporary server issues: Server hardware or software issues, network problems, or other transient issues can also lead to SMTP error 451.

How to fix SMTP Error 101?

  • Wait and retry: In many cases, SMTP error 451 is a temporary issue, and the best course of action is to wait and retry sending the email later. The receiving server may have been overloaded or temporarily unavailable, and the problem may resolve itself.
  • Check for greylisting: If you suspect greylisting is causing the error, simply waiting and retrying the email should resolve the issue when the receiving server accepts the email upon the retry.
  • Verify rate limits: If you are sending a high volume of emails, make sure you are not exceeding any rate limits imposed by the receiving server. If you are, consider spreading out your email sending over a longer period.
  • Investigate server issues: If you are consistently encountering SMTP error 451 when sending emails to a specific recipient, it's possible that the recipient's mail server is experiencing server issues. You may want to contact the recipient's email administrator for assistance.

SMTP Error 101 Examples:

Example 1: "451 Temporary failure, please try again later."

Example 2: "451 Greylisted - Try again in 15 minutes."

Example 3: "451 Rate limit exceeded for Try again in an hour."

Example 4: "451 Service temporarily unavailable due to server maintenance. Please retry later."