Yahoo SMTP Error Codes
Complete guide to Yahoo Mail SMTP errors. Understand TSS errors, authentication failures, and policy blocks to improve your email deliverability to Yahoo.
📧 About Yahoo Mail
Yahoo Mail was launched in 1997 as an extension of Yahoo's web portal. At launch, it offered a significant advantage with 4 MB of storage capacity, much larger than competitors. Despite challenges and competition from services like Gmail, Yahoo Mail remains one of the popular email providers globally.
⚠️ Important Yahoo Updates
June 2025 Migration - Comcast & AT&T
Comcast.net Migration
Phasing out legacy Xfinity mail platform and progressively migrating user accounts to Yahoo Mail. Users retain comcast.net addresses along with folders, contacts, and content. They'll receive phased notifications and must accept Yahoo's terms when prompted. After migration, mail access continues via Yahoo's web, app, or IMAP/SMTP servers.
AT&T Domains Migration
As of late June 2025, AT&T started updating MX records for consumer domains to point directly to Yahoo's inbound mail servers: mx-att.mail.am0.yahoodns.net
Affected domains: ameritech.net, att.net, bellsouth.net, currently.com, flash.net, nvbell.net, pacbell.net, prodigy.net, sbcglobal.net, snet.net, swbell.net, wans.net and worldnet.att.net
Note: Delivery issues at Prodigy.net hosted domains, previously addressed through [email protected] will need to be addressed via Yahoo's support pages on the Sender Hub.
Q1 2024 Requirements (Now In Effect)
Starting Q1 2024, Yahoo requires all bulk senders to:
- Authenticate email with SPF, DKIM, and DMARC
- Enable easy unsubscription - one-click unsubscribe (RFC 8058)
- Honor unsubscribe requests within 2 days
- Only send emails users want - maintain spam complaint rates below threshold
Yahoo will provide more details about these new criteria. Meeting these requirements improves email delivery.
Yahoo SMTP Error Codes Explained
421 4.7.0 [TSS04]
Temporary DeferralMessages from xx.xx.xx.xx temporarily deferred due to unexpected volume or user complaints - 4.16.55.1
This error typically occurs when the sender has a poor reputation or no sending history. See https://postmaster.yahooinc.com/error-codes
How to Fix:
- ✓Reduce sending volume and warm up your IP slowly
- ✓Check for user complaints via Yahoo's Complaint Feedback Loop
- ✓Verify your authentication (SPF, DKIM, DMARC) is properly configured
- ✓Review your list quality and remove inactive or invalid addresses
451
Temporary FailureMessage temporarily deferred due to unresolvable From header domain xxx.com
Yahoo cannot resolve the domain in your From header. This is a temporary failure that may resolve itself. See https://senders.yahooinc.com/error-codes#invalid-from
How to Fix:
- ✓Ensure your From domain has valid DNS records
- ✓Check that your domain hasn't expired
- ✓Verify DNS propagation if recently changed
- ✓Use a fully qualified domain name in the From header
554
Permanent Failure - From DomainMessage permanently deferred due to unresolvable From header domain xxx.com
The From address or domain is invalid and Yahoo has permanently rejected the message. See https://senders.yahooinc.com/error-codes#invalid-from
How to Fix:
- ✓Fix the From domain to use a valid, resolvable domain
- ✓Ensure the From address is properly formatted
- ✓Don't use generic domains like @localhost or @example.com
- ✓Verify your sending domain exists and has MX records
554
Permanent Failure - RFC.5321Message permanently deferred due to unresolvable RFC.5321 from domain
The envelope From (Return-Path) domain is invalid according to RFC 5321 standards. See https://senders.yahooinc.com/error-codes#unresolvable-from-domain
How to Fix:
- ✓Ensure your Return-Path domain is valid and resolvable
- ✓Match your Return-Path domain with your sending domain
- ✓Configure your mail server to use proper envelope addresses
- ✓Check for typos in your mail server configuration
554
Authentication FailureThis mail has been blocked because the sender is sending too much mail for its authentication status
Yahoo requires proper authentication for bulk senders. Please set up DKIM aligned to the From: domain. See https://senders.yahooinc.com/smtp-error-codes#authentication-failures for more information.
How to Fix:
- ✓Set up DKIM aligned to the From: domain immediately
- ✓Implement SPF records for your sending IPs
- ✓Configure DMARC policy for your domain
- ✓Reduce volume until authentication is properly configured
554 5.7.9
Policy ViolationMessage not accepted for policy reasons
Your message violates Yahoo's sending policies. This can be due to content, authentication, or header formatting issues. We've seen such bounces when special characters in the Subject line header are not properly encoded. See https://senders.yahooinc.com/error-codes
How to Fix:
- ✓Review Yahoo's sending guidelines and policies
- ✓Check for improperly encoded special characters in headers
- ✓Ensure all headers are correctly formatted
- ✓Verify your content doesn't trigger spam filters
- ✓Fix any authentication issues (SPF/DKIM/DMARC)
552 1
Invalid RecipientRequested mail action aborted, mailbox not found
The bounce message refers to a non-existing recipient. Such recipients should be treated as a hard bounce, not be retried and removed immediately from the mailing list after the first occurrence.
How to Fix:
- ✓Remove this recipient from your mailing list immediately
- ✓Do not retry sending to this address
- ✓Implement proper bounce handling in your system
- ✓Use double opt-in to verify addresses before adding to lists
554 30
Disabled MailboxSorry, your message to *@yahoo.com cannot be delivered. This mailbox is disabled (554.30)
This indicates that you're sending to an inactive mailbox. Hitting a high number of such mailboxes indicates bad list quality.
How to Fix:
- ✓Remove inactive addresses from your list
- ✓Implement re-engagement campaigns before addresses go stale
- ✓Use email validation services before sending
- ✓Monitor engagement and remove non-responders
554 [PH01]
Policy BlockMessage not allowed - Email not accepted for policy reasons
This indicates a permanent failure triggered by DMARC failure, Spamhaus listings or various other Yahoo policies that prevent your emails from being delivered. Please visit https://postmaster.yahooinc.com/error-codes
How to Fix:
- ✓Check if your IP/domain is listed on Spamhaus
- ✓Verify DMARC alignment and policy settings
- ✓Review Yahoo's bulk sender requirements
- ✓Contact Yahoo support if the issue persists
553 5.7.2 [TSS09]
Permanent BlockAll messages from xx.xx.xx.xx will be permanently deferred; Retrying will NOT succeed
This indicates that Yahoo permanently blocked your sending IP/domain. You should first find out why this happened, mitigate the issue on your end and then try to get unblocked by reaching out to Yahoo support. See https://postmaster.yahooinc.com/error-codes
How to Fix:
- ✓Identify and fix the root cause of the block
- ✓Check for compromised accounts or malware
- ✓Review sending practices and list quality
- ✓Contact Yahoo support through their postmaster site
- ✓Consider using a different IP or email service provider
553 5.7.2 [TSS11]
Permanent BlockAll messages from xx.xx.xx.xx will be permanently deferred; Retrying will NOT succeed
This indicates that Yahoo permanently blocked your sending IP/domain. Similar to TSS09, retrying will not succeed. See https://postmaster.yahooinc.com/error-codes
How to Fix:
- ✓Stop sending immediately to prevent further reputation damage
- ✓Audit your entire email program for issues
- ✓Implement proper authentication (SPF, DKIM, DMARC)
- ✓Work with Yahoo support to resolve the block
- ✓Consider switching to a reputable ESP if self-hosting
Yahoo Email Best Practices
Authentication Requirements
- •Implement SPF, DKIM, and DMARC for all sending domains
- •Ensure DKIM is aligned with your From domain
- •Set up proper reverse DNS (PTR) records
- •Use consistent From addresses and domains
Anti-Spam Measures
- •Yahoo uses Spamhaus blocklists
- •SURBL for URL reputation checking
- •Public RBL integration
- •Monitor your listings regularly
List Management
- •Use double opt-in for new subscribers
- •Remove hard bounces immediately
- •Implement re-engagement campaigns
- •Regularly clean your list of inactive users
Sending Practices
- •Warm up new IPs gradually
- •Limit to 20 emails per connection
- •Open new connections for additional emails
- •Monitor complaint rates via Yahoo CFL
Content Guidelines
- •Provide clear, one-click unsubscribe options
- •Avoid spam trigger words and excessive punctuation
- •Properly encode special characters in headers
- •Maintain good text-to-image ratios
Yahoo-Specific Features & Programs
📧 Complaint Feedback Loop (CFL)
Yahoo's CFL is domain-based (no longer IP/CIDR-based) and requires DKIM signing. Enroll to receive notifications when recipients mark your emails as spam. Postmastery is fully integrated with Yahoo's CFL program through its monitoring tools.
⚠️ Important: Yahoo decommissioned their old FBL at the end of 2024. All senders must re-enroll through the new interface or they will stop receiving complaints.
⏰ View Time Optimization (VTO)
VTO ensures your emails appear at the top of the inbox when recipients check their mail, regardless of when sent. This can significantly increase engagement rates.
📊 Deliverability & Performance Feeds
Yahoo offers paid feeds similar to Google Postmaster Tools:
- • Placement Feed: Inbox vs spam metrics
- • Performance Feed: Opens, clicks, deletes by campaign
✅ Yahoo's Complete Best Practices Checklist
Yahoo suggests the following best practices to optimize delivery:
- ✓Send emails customers want
- ✓Authenticate using SPF, DKIM, and DMARC
- ✓Segregate email types by IP or DKIM domain
- ✓Use OPT-IN method to confirm user subscriptions
- ✓Allow users to easily unsubscribe
- ✓Remove invalid recipients
- ✓Enroll in complaint feedback loop (CFL)
- ✓Publish reverse DNS (PTR) records
- ✓Ensure mail servers are secure
- ✓Control the flow of your outbound emails
- ✓Be compliant with RFCs and CAN-SPAM
Technical Configuration Tips
Yahoo SMTP Connection Limits
MAX_MESSAGES_PER_CONNECTION = 20
if message_count >= 20:
close_connection()
open_new_connection()
# Allows CFL tracking without re-enrolling for each new domain
DKIM-Signature: v=1; a=rsa-sha256; d=yourdomain.com ...
DKIM-Signature: v=1; a=rsa-sha256; d=sendingdomain.com ...
Yahoo Resources & Support
Yahoo Sender Hub
Official documentation, best practices, and sender resources
Error Code Reference
Yahoo's official SMTP error code documentation
Yahoo Support
Individual senders experiencing messages going to spam or missing can report false-positives through the "Problems Delivering Mail" section
You'll receive an automated reply and may need to continue responding to the email thread until resolved. Yahoo support has considerably improved recently - you are quite likely to resolve delivery issues if you follow best practices.
Still Having Yahoo Delivery Issues?
If you're experiencing persistent Yahoo delivery problems or need expert help implementing these fixes, we can analyze your specific situation and provide tailored solutions.