Yahoo

Known for its freemail services AOL and Yahoo, they provide a detailed Postmaster page with contact options for investigating false-positive spam verdicts. They offer a Senders Hub Insights Dashboard, as well as a Complaint Feedback Loop, more details can be found on the link below. As of August 2025, they also handle the incomming connections for the AT&T domains.

Links

Domains

Codes

421 · 450 · 451 · 452 · 501 · 550 · 552 · 553 · 554

421↑
421 Service not available, closing transmission channel

This indicates that the domain is not currently accepting email.

421 Max message per connection reached, closing transmission channel

You have exceeded the number of messages you can send in a single connection. If you do not control the MTA, you may need to ask your email provider for adjustments.

421 4.7.0 [TSS04] Messages from 0.0.0.0 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1

This could indicate one of few problems, either that your emails are generating an excessive amount of complaints from Yahoo users or Yahoo is seeing unsual traffic patterns from your mail servers. It could also mean that your email content contains objectionable content or show characteristics indicative of spam. Another possiblity is that your IP has a poor reputation

450↑
450 User is receiving mail too quickly

The email address is currently receiving an email at a high rate. Re-sending email in the future will most likely be successful.

451↑
451 example.yahoo.com Resources temporarily unavailable. Please try again later [#4.16.1].

The sender's domain is currently unable to accept this message. Re-sending email in the future will most likely be successful.

451 4.3.2 Internal server error

The mail server encountered a temporary failure. Most likely it isn't anything to do with the session, but rather the server is overloaded or having issues. If you try again later, it should work.

452↑
452 temporary domain failure <example@example.com>

The mail server encountered a temporary failure. Most likely it isn't anything to do with the session, but rather the server is overloaded or having issues. If you try again later, it should work.

501↑
501 Syntax error in parameters or arguments

It usually happens when a server cannot read the command you sent because of invalid characters, missing domains, or improper spacing. Review how the message is structured and try again.

550↑
550 relaying denied for <example@example.com>

This domain is not hosted at Yahoo. There is little you can do, as it is likely a misconfiguration on the receipient's end.

552↑
552 1 Requested mail action aborted, mailbox not found

Treat this bounce as a 'bad-mailbox'. Experience has shown that the email address is invalid and should not be sent to in the future.

553↑
host xx.yahoodns.net [x.xx.xx.xx] SMTP error from remote mail server after MAIL FROM:<user@example.com> SIZE=2022: 553 5.7.1 [BL21] Connections will not be accepted from x.xx.xx.xx, because the ip is in Spamhaus's list

Your IP is listed in the Spamhaus ZEN list. Spamhaus is a very reputable block list with few false-positive listings. Use the link below to check the status of your IP. You must cooperate with Spamhaus to resolve the listing. Remember that even once the listing is removed, it may take receivers up to 24 hours to refresh their cache and to see delivery resume.

host xx.yahoodns.net [x.xx.xx.xx] SMTP error from remote mail server after MAIL FROM:<user@example.com> SIZE=2022: 553 Mail from x.xx.xx.xx not allowed - 5.7.1 [BL23] Connections not accepted from IP addresses on Spamhaus XBL [550]

Your IP is listed in the Spamhaus XBL list. Spamhaus is a very reputable block list with few false-positive listings. Use the link below to check the status of your IP. You must cooperate with Spamhaus to resolve the listing. Remember that even once the listing is removed, it may take receivers up to 24 hours to refresh their cache and to see delivery resume.

554↑
554 5.7.9 This mail has been blocked because it failed authentication checks against the sending domains DMARC policy.

The sent message failed the DMARC policy for the sending domain. If the sending domain has a reject policy set for DMARC, the message must pass DKIM or have SPF aligned to the domain with a custom Return-Path. To resolve this, you'll need to contact your email service provider to configure how to send a message that passes DMARC.

554 delivery error: dd Sorry, your message to example@example.com cannot be delivered. This mailbox is disabled (554.30). - example.yahoo.com

The message of the reciever is disabled. It may come back at somepint, or it may not. The best option is suppress this email address from your lists, as it hasn't used its inbox in a very long time.

554 delivery error: dd Requested mail action aborted - example.yahoo.com
554 5.7.9 Message not accepted for policy reasons.

This rejection occurs out of suspicion that the message or sender is not trustowrthy. Review the URLs and images in your content. Once investigated and you can verify your messages are safe, you can request a review from the Yahoo Postmaster Support team.

554 Message not allowed - [PH01] Email not accepted for policy reasons.

This rejection occurs out of suspicion that the message or sender is phishing. Review the URLs and images in your content. Once investigated and you can verify your messages are safe, you can request a review from the Yahoo Postmaster Support team.

554 Message not allowed - Headers are not RFC compliant [291]

Your message headers are not compliant. Double check your sending settings and parameters or ask your email provider for assistance.