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 abuse_rbl@abuse-att.net 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 [TS01]
Temporary DeferralYahoo deferred the connection because the sending IP has little or no history.
TS01 appears during IP or domain warmup. Yahoo pauses delivery until it sees strong engagement and consistent authentication.
How to Fix:
- ?Warm new IPs with small batches of highly engaged recipients.
- ?Distribute sends over several hours instead of one burst.
- ?Keep From domains, DKIM selectors, and envelope senders consistent.
- ?Monitor Yahoo complaint feedback loop results daily while ramping.
421 4.7.0 [TS02]
Temporary DeferralMail is paused because Yahoo is seeing a spike in spam complaints.
TS02 reflects a trust downgrade driven by recent complaint or unsubscribe activity. Yahoo slows delivery so recipients are not overwhelmed.
How to Fix:
- ?Suppress unengaged contacts who have not opened in the past 90 days.
- ?Verify list acquisition sources and drop any third-party data.
- ?Highlight the unsubscribe link and confirm it works within one click.
- ?Pause campaigns that triggered the spike until reputation rebounds.
421 4.7.0 [TS03]
Temporary DeferralTraffic is temporarily held while Yahoo reviews a potential abuse signal.
TS03 is triggered by spikes in unknown users, sudden content changes, or infrastructure anomalies that resemble abuse.
How to Fix:
- ?Audit recent creative, link domains, and sending speeds for big shifts.
- ?Ensure bounce processing removes invalid Yahoo addresses immediately.
- ?Confirm no other teams or systems are sending through the same IPs.
- ?Resume at a slower pace after the issue is corrected.
421 4.7.0 [TSS04]
Temporary DeferralYahoo deferred further messages because the IP is sending more than its reputation allows.
TSS04 is the classic warmup throttle. Yahoo slows deliveries when an IP expands too quickly or after complaint spikes.
How to Fix:
- ?Throttle campaigns and resume with smaller hourly caps.
- ?Check complaint feedback loop and unsubscribe processing for issues.
- ?Verify SPF, DKIM, and DMARC alignment for every domain in use.
- ?Scrub recent imports or low-engagement cohorts before resuming.
451
Temporary FailureYahoo could not resolve the visible From domain and deferred the message.
A missing DNS zone, expired domain, or incomplete propagation can trigger this soft bounce. Once DNS is healthy, Yahoo accepts the traffic.
How to Fix:
- ?Verify A, MX, and TXT records for the From domain resolve globally.
- ?Confirm the domain registration is active and not in redemption.
- ?Avoid newly delegated domains until DNS propagation finishes.
- ?Fallback to a proven From domain while the issue is resolved.
554
Permanent Failure - From DomainYahoo rejected the message because the From domain does not resolve.
If DNS for the visible From domain is missing or misconfigured, Yahoo blocks the message instead of retrying later.
How to Fix:
- ?Restore authoritative DNS records for the From domain immediately.
- ?Retire placeholder or test domains from production sends.
- ?Monitor DNS expiry dates and automate renewal reminders.
- ?Send from a secondary domain while DNS caches refresh.
554
Permanent Failure - RFC 5321Yahoo could not resolve the envelope From (Return-Path) domain.
If the bounce domain in the envelope From lacks DNS or authentication alignment, Yahoo interprets it as spoofing and rejects the mail.
How to Fix:
- ?Publish A/AAAA and MX records for the bounce domain.
- ?Ensure SPF covers the Return-Path host and matches the visible From domain.
- ?Double-check the MTA configuration for typos or legacy hostnames.
- ?Avoid mixing ESP-provided Return-Paths with self-hosted IPs.
554
Authentication FailureYahoo rejected the campaign because SPF and DKIM did not pass at this volume.
High-volume senders must pass SPF and DKIM with DMARC alignment. Continued failures cause Yahoo to bounce rather than defer messages.
How to Fix:
- ?Sign every message with DKIM aligned to the visible From domain.
- ?Limit SPF to the IP ranges that are actively sending mail.
- ?Publish a DMARC policy and monitor aggregate reports for gaps.
- ?Reduce daily volume until authentication passes consistently.
554 5.7.9
Policy ViolationYahoo blocked the message because it violated sending policies or formatting rules.
Improperly encoded subject lines, malformed headers, or risky content can trigger 5.7.9. It is often a signal to review MIME structure and compliance.
How to Fix:
- ?Re-encode subject lines and headers using proper MIME or UTF-8 encoding.
- ?Review message copy for phrases commonly associated with spam.
- ?Confirm authentication passes and that DKIM canonicalization matches the message body.
- ?Test the campaign with deliverability tools before resending.
552 1
Invalid RecipientThe mailbox referenced in the bounce does not exist at Yahoo.
Hard bounces from invalid accounts should be removed immediately. Continuing to target them damages domain and IP reputation.
How to Fix:
- ?Drop the address after the first 552 bounce; do not retry.
- ?Enable automated bounce handling in your ESP or MTA.
- ?Require confirmed opt-in for new Yahoo signups.
- ?Audit data entry or import pipelines for formatting mistakes.
554 30
Disabled MailboxDelivery failed because the Yahoo mailbox is disabled or dormant.
Yahoo deactivates mailboxes after long periods of inactivity. Repeated attempts imply poor list hygiene and can hurt future inboxing.
How to Fix:
- ?Suppress addresses that bounce with 554.30 and launch a reactivation workflow.
- ?Run regular engagement sweeps to remove dormant Yahoo accounts.
- ?Use validation services on legacy lists before periodic sends.
- ?Monitor inactivity trends so they do not impact major launches.
554 [PH01]
Policy BlockYahoo blocked the campaign because of a policy, blocklist, or DMARC issue.
PH01 often follows Spamhaus listings, DMARC failures, or compromised infrastructure. Yahoo expects proof of remediation before traffic resumes.
How to Fix:
- ?Check public and private blocklists for the sending IP and domain.
- ?Resolve DMARC alignment issues and publish a policy with reporting.
- ?Document corrective actions and share them with Yahoo Sender Support.
- ?Resume with low volume and monitor complaint rates closely.
553 5.7.2 [TSS09]
Permanent BlockYahoo permanently deferred all messages from this IP or domain. Retrying will not work.
TSS09 indicates Yahoo saw significant abuse or policy violations. You must fix the root cause and provide evidence before delisting.
How to Fix:
- ?Stop sending immediately and audit infrastructure for compromise.
- ?Resolve complaint, trap, and bounce issues across every list source.
- ?Submit a detailed mitigation request through the Yahoo Sender Hub.
- ?When approved, restart with small, highly engaged segments.
553 5.7.2 [TSS11]
Permanent BlockYahoo permanently blocked the sender. Retrying without remediation will fail.
TSS11 usually follows spam complaints, compromised accounts, or repeated violations of Yahoo’s bulk sender policies.
How to Fix:
- ?Halt traffic and investigate every stream that sends to Yahoo domains.
- ?Close security gaps and document the fixes in a mitigation plan.
- ?Provide proof of remediation to Yahoo support before resuming.
- ?Send small, monitored batches once Yahoo confirms the unblock.
554 5.7.1 [TS06]
Permanent BlockYahoo blocked the message because the sender reputation remains too low.
TS06 is typically issued after repeated TS02 or TS03 warnings. Yahoo expects proof that complaint and bounce levels are under control.
How to Fix:
- ?Pause Yahoo sends and tackle complaint, bounce, and trap metrics.
- ?Ensure no secondary systems or compromised accounts are sending mail.
- ?Share remediation steps with Yahoo before requesting reinstatement.
- ?Reintroduce traffic with a small, high-engagement cohort once cleared.
554 5.7.1 [TS07]
Policy BlockYahoo rejected the campaign because the content or URLs look abusive.
TS07 is often tied to phishing-like copy, obfuscated redirects, or malware hosting. Yahoo expects the offending content to be removed.
How to Fix:
- ?Review the exact creative that triggered the bounce and remove risky wording or links.
- ?Scan landing pages and tracking domains with multiple reputation tools.
- ?Avoid URL shorteners or chained redirects that obscure destination domains.
- ?Provide screenshots and remediation notes when contacting Yahoo support.
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 confirmed opt-in for new Yahoo subscribers
- Allow users to unsubscribe in a single click
- Remove invalid recipients immediately
- Enroll in the Yahoo complaint feedback loop (CFL)
- Publish reverse DNS (PTR) records for every IP
- Ensure MTAs enforce TLS and modern ciphers
- Control throughput so Yahoo traffic ramps gradually
- Follow RFC and CAN-SPAM requirements
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.