SMTP Error Codes


An SMTP error, or Delivery Status Notification (DSN), indicates that an email could not be delivered, either due to a temporary or permanent problem. You can review our list of SMTP error messages below for details about each error.


What are 4XX (421 and 451) temporary errors?
  • A 421 or 451 SMTP error indicates a temporary problem blocking the delivery of your message. Message delivery can be delayed for multiple reasons including:
    • 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 users.
    • The Yahoo mail servers are busy when attempting a connection.
    • Other suspicious behavior which leads Yahoo to enforce a dynamic deprioritization for your sender's server.
  • Review our Sender Requirements & Recommendations to learn how you can help your emails reach Yahoo.
  • If you're not the administrator of the mail server in question, we suggest you contact the administrator directly to discuss the error message you're receiving.
  • If you encounter 4XX errors when trying to send an email to Yahoo, you may retry sending at a later time.
What are 5XX (553 and 554) permanent errors?
  • A 553 or 554 SMTP error indicates an email could not be delivered due to a permanent problem. Message delivery can be permanently deferred because:
    • You're trying to send a message to an invalid email address.
    • Your message failed authentication checks against your sending domain's DMARC or DKIM policy.
    • The message contains characteristics that Yahoo won't accept for policy reasons.
    • Other suspicious behavior which leads Yahoo to issue a permanent rejection for your SMTP connection.
    • Your IP is listed by Spamhaus. Please check with https://www.spamhaus.org.
  • If you consistently receive 5xx errors when sending to Yahoo, we encourage you to review our Sender Requirements & Recommendations, since 5xx errors can be a symptom of a more widespread, general problem.
  • You should not retry sending an email that comes back with with a 5xx error. List managers should have a policy for removing email addresses that generate 5xx errors/bounces.
RFC compliance failures
  • These errors indicate emails from your mail server are not RFC compliant.
  • See our Sender Requirements & Recommendations for more information about RFC compliance.
  • These failures can include:
    • Duplicate headers
    • Incorrectly formatted headers
    • Incorrect mime types
    • …and more
Unresolvable RFC.5321 or RFC.5322 from domain
  • These errors indicate that the domain used to the right of the @ in the MAIL FROM or HEADER FROM does not appear to be valid.
  • We determine if the domain name exists by using an SOA query; therefore, if multiple subdomains are used in MAIL FROM or HEADER FROM commands, then besides setting up a DNS A or MX record (perhaps using a wildcard), then SOA records must be set up as well.
  • These errors can be returned as either a timeout/servfail (451) or a permfail (554)
Excessive user complaints
  • These errors indicate emails from your mail server are generating an excessive amount of complaints from Yahoo users. We suggest you consider the following:
    • Monitor your sender reputation - Even if you have a good reputation, users can vote your email as spam and affect your overall reputation.
    • If you're an administrator of message content and mailing policy and you've deployed significant changes or you've received this error for more than 48 hours, we ask that you review your outgoing messages for objectionable content or practices.
    • Using a shared IP address - Mail traffic from other domains could be negatively affecting your IP sending reputation. If applicable, contact your host provider to request using dedicated IP addresses to send your mail to resolve this problem.
  • We recommend you review our Sender Requirements & Recommendations to ensure you're following proper opt-in methods of user subscriptions.
Excessive unsolicited messages
  • Excessively high volume of emails from a single IP address is a characteristic of unsolicited, bulk emailing. Don't resend the email until you review our Sender Requirements & Recommendations and make any necessary changes.
  • You may have setup the IP you are using to send email very recently and did not increase the email traffic slowly.
  • Check your subscription practices and lists to ensure that messages are sent only to users who've requested it.
  • If you're an administrator for message content or mailing policy and you've made significant changes, or you've received this error for more than 48 hours, we ask that you review your outgoing emails for objectionable content or practices.
Excessive unknown recipients
  • Your mail server is sending to a large number of invalid recipients and may be configured as an “open relay” or “open proxy”.
  • Open proxies and open relays are a very common source of spam and Yahoo doesn't accept email from them.
  • We encourage you to follow our Sender Requirements & Recommendations and make appropriate changes to secure your servers.
  • Review your mailing lists and remove any addresses that generate bounces.
  • Examine your outbound queues for potentially objectionable content to ensure that spammers aren't abusing your mail server.
Authentication failures
  • Your email failed one or more authentication checks that Yahoo uses to verify emails are truly sent from the domains they claim to originate from.
  • All senders need to authenticate their emails with either SPF or DKIM. Bulk senders must authenticate with SPF and DKIM, and have a DMARC policy in place (see our Sender Requirements & Recommendations section for more information).
  • Yahoo rejects emails for failing DKIM authentication when all of following conditions apply:
    • The signing domain publishes a policy which states that all emails from the domain must be signed and authenticated with DKIM to prevent forgery.
    • The signing domain is identified in the “d=” tag of the DKIM signature.
    • The rejected email couldn't be authenticated against the sending domain's policy, for example, due to a missing or bad signature.
  • If you're not the system administrator for the mail servers affected, we encourage you to contact the administrator, so they can look into the situation further.
  • If you are sending your email via Microsoft365 or Google Workspace, please follow these guidelines to properly authenticate your emails via DKIM: Google help article, Microsoft help article.
  • For mailing lists, also known as “listservs,” you should change your sending behavior by adding the mailing lists' address to the “From:” line, rather than the sender's address. Also, enter the actual user/sender address into the “Reply-To:” line.
  • If your mail is forwarded, consider adding ARC (Authenticated Received Chain).
  • It's a good idea to review our guide to Sender Requirements & Recommendations and make appropriate changes.
Content based blocks (e.g: PH* Errors)
  • These error messages indicates that your email wasn't accepted because there is something in the content that Yahoo won't accept for policy reasons.
  • Objectionable content that Yahoo deems unacceptable includes:
    • Viruses
    • Phishing attempts
    • Ransomware
    • Other malicious software
    • Links or URLs to any of the above
  • Examine your outbound queues for potentially objectionable content to ensure that spammers aren't abusing your mail server.
  • Follow our Sender Requirements & Recommendations and make appropriate changes to secure your servers.
Recipient does not exist
  • The Yahoo account that you're trying to send to does not exist. We recommend that you contact the recipient directly to confirm their correct email address.
  • You should not retry delivery of the message. It will never complete successfully.
  • You should remove the email address from your mailing list.
Message temporarily deferred (eg: TS* Errors)
  • This is a temporary error and your mail server may automatically re-try sending the email at a later time. Your message may have been deferred due to one or more of the following:
    • Emails from your mail server are generating substantial complaints from Yahoo users.
    • The message contained objectionable content or exhibited characteristics indicative of spam.
    • The IP (x.x.x.x or its subnet, i.e., .255 ) has a poor reputation.
    • We are seeing unusual traffic patterns from your mail servers.
  • We ask that you follow our Sender Requirements & Recommendations and review your outgoing messages for potentially objectionable content.
  • If your mail server does not primarily send bulk mailings (e.g., you run a personal, corporate, educational, or ISP mail server), examine your outbound queues to ensure that spammers aren't abusing your mail server.
  • If you're seeing the same error consistently over an extended period of time, we encourage you to submit a Sender Support Request with specific details of the error and diagnostic codes you see in your logs.
Resources temporarily unavailable
  • This error indicates that the Yahoo mail servers were busy and temporarily unable to process your transaction at the time of connection.
  • Such issues are generally brief, and normal connectivity is usually restored after a short period of time.
Sample Email failed delivery message
  • When you get mail from a “MAILER-DAEMON” or a “Mail Delivery Subsystem” with a subject similar to “Failed Delivery,” this means that a message you sent was undeliverable and has been bounced back to you.
  • These messages are produced automatically and usually include a reason for the delivery failure.
  • Failed or “bounced” messages normally consist of two parts:
    • The reason for the bounce
    • Your original message
  • Failure notice example:

Hi. This is the qmail-send program at yahoo.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

123.45.67.89 failed after I sent the message.
Remote host said: 554 delivery error: dd This user doesn't have a yahoo.com account (testing123@yahoo.com) [-5] - mta123.mail.mud.yahoo.com

--- Below this line is a copy of the message.

Return-Path:
Received: (qmail 24519 invoked by uid 60001); 11 Nov 2010 15:37:25 -0000
...
...
X-Mailer: YahooMail/11.4.9 YahooMailWebService/0.8.107.285259
Date: Thu, 11 Nov 2010 07:37:24 -0800 (PST)
From: Yahoo Customer Care
Subject: Test Message
To: testing123@yahoo.com
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary=“0-162906830-1289489844=:24510”

--0-162906830-1289489844=:24510
Content-Type: text/plain; charset=us-ascii

[Text of the original message here]

--0-162906830-1289489844=:24510--

The most common reason for a failed delivery is that the email address you entered isn't valid. If the Mailer Daemon says that the account doesn't exist, double-check the spelling of the address you entered. A single misplaced letter can be enough to cause a failed delivery. If the message continues to be bounced back to you, you might want to verify that the account hasn't been closed or moved.