4xx SMTP error codes

4xx SMTP errors indicate a temporary problem blocking the delivery of your message. Your system should retry sending messages that aren't delivered automatically. If not, you can try to resend the message on your own 4 hours after you receive the error. Message delivery can be delayed because:

  • We've observed unusual traffic patterns from your sending server's IP address.
  • The message you attempted to send included characteristics of spam.
  • Emails from your mail server have been generating complaints from Yahoo Mail users.
  • The Yahoo Mail servers are busy when attempting a connection.

Our recommendations

If you're not the administrator of the mail server in question

Contact your mail server's administrator to discuss the error.

If you're a message content author or mailing policy administrator

If you've made significant changes to resolve the error, but the error still remains consistent over a 48-hour period, we encourage you to review our recommended best practices when sending to Yahoo Mail.

  • List managers should have a policy for removing email addresses that generate errors/bounces.
  • We review IPs submitted for delivery issues and, based on our review, we may modify your reputation in our systems if needed.
    • Fill out a new sender application if your email is being deferred by Yahoo. This doesn't guarantee inbox delivery.
    • Even if you have a good reputation, users can vote your email as spam and affect your overall reputation.
    • Examine your outbound queues to ensure that spammers aren't abusing your mail server.
    • If you’re using a shared IP address, mail traffic from other domains could be negatively affecting your IP sending reputation. This traffic can result from things outside the sender's control.
  • Consider the following when you review outgoing messages for objectionable content or practices:

Learn more about bulk mail sender reputation

 Good to know: Yahoo is unable to disclose specific information regarding our filtering practices.

421 SMTP error codes

If you regularly receive 421 SMTP errors when sending email to Yahoo Mail addresses, check your server logs for unusual traffic patterns or objectionable message content.

421 4.7.0 [TS01] Messages from x.x.x.x temporarily deferred due to user complaints - 4.16.55.1

This error indicates emails from your mail server have unusual traffic patterns or are generating complaints from Yahoo Mail users.

421 4.7.0 [TS02] Messages from x.x.x.x ... deferred due to excessive user complaints - 4.16.56.1

This error indicates emails from your mail server have unusual traffic patterns or are generating complaints from Yahoo Mail users. This error is usually received by senders on a shared IP.

421 4.7.1 [TS03] All messages from x.x.x.x permanently deferred

The error message indicates Yahoo is seeing a high volume of emails from your IP address, which is a characteristic of unsolicited, bulk emailing.

If you're seeing this message, don't resend the email until you check your subscription practices and lists to ensure that messages are sent only to users who've requested it.

421 Message temporarily deferred - [numeric code]

The [numeric code] shows a specific diagnostic code (e.g., "4.16.51", "4.16.52") in your SMTP logs which relates to the reason your email was deferred, and is related to:

  • Low connectivity percentage to Yahoo MTAs.
  • Bad tokens within the message.
  • Unusual message traffic containing PDF files.

421 Resources temporarily unavailable. Please try again later

The Yahoo Mail MTAs are temporarily too busy. Your server should try to resend the message after a few minutes.

451 SMTP error codes

If you consistently see 451 SMTP error codes, review your mailing list management practices and policies.

451 Message temporarily deferred - [numeric code]

The [numeric code] shows a specific diagnostic code (e.g., "170", "200") in your SMTP logs, and is related to:

  • A URL that's been flagged for excessive traffic.
  • An IP data query failure.

451 Resources temporarily not available - Please try again later [#4.16.5]

The Yahoo Mail servers are temporarily too busy. Your server should try to resend the message after a few minutes.

451 VS1-IP Excessive unknown recipients - (#4.1.8)

Your mail server is configured as an open relay or proxy. We encourage you to examine your outbound queues to ensure that spammers aren't abusing your mail server. Review our guidelines for avoiding deprioritization.

451 VS1-MF Excessive unknown recipients - possible Open Relay (#4.4.5)

The sender's MAIL FROM address has been sending to excessive bouncing accounts (non-active, don't exist). Review your mailing lists, and remove any addresses that generate bounces. Review our guidelines for avoiding deprioritization.