Why are Yahoo open rates dropping and what are TSS04 and TSS05 errors?
Matthew Whittaker
Co-founder & CTO, Suped
Published 26 Jul 2025
Updated 15 Aug 2025
8 min read
Over the past few months, many of us sending emails to Yahoo and AOL recipients have noticed a concerning trend: a significant drop in open rates. It's not just a minor fluctuation; we're talking about substantial decreases, even for highly engaged lists and transactional mail. This widespread issue has left many of us wondering what exactly is going on.
The problem often manifests beyond just lower open rates. Many senders are also encountering an increase in bounce messages, specifically those citing TSS04 and TSS05 errors. These technical bounce codes, originating from Yahoo's mail servers, indicate specific issues that are preventing emails from reaching the inbox, or even from being temporarily deferred.
This isn't an isolated incident affecting a few unlucky senders. It appears to be a systemic change, with many providers reporting similar patterns. The shift seems to stem from adjustments in Yahoo's email policies, particularly concerning how they assess sender reputation.
This article explores the reasons behind these dropping open rates and the significance of TSS04 and TSS05 errors. We'll look at the causes and outline practical steps you can take to mitigate the impact and restore your email deliverability with Yahoo (and its partner domains like AOL and AT&T).
The evolving landscape of Yahoo's email policies
Yahoo has been making significant changes to its mail policies, moving away from primarily relying on IP reputation to focusing almost entirely on domain reputation. This shift impacts how your emails are filtered and can directly affect open rates. If your domain's reputation is suffering, emails are more likely to land in spam folders or be blocked entirely, even if your IP address is pristine.
The focus on domain reputation means that consistent authentication via SPF, DKIM, and DMARC is more crucial than ever. Yahoo (and Google) now require senders of over 5,000 emails per day to implement robust email authentication to ensure deliverability. Failing to meet these standards can lead to severe deliverability issues, including the TSS04 and TSS05 errors we are seeing.
We've also observed that Yahoo is becoming more aggressive in filtering out unwanted mail, especially bulk emails. This includes a heightened sensitivity to user complaints, spam trap hits, and engagement metrics. A low engagement rate or a sudden spike in spam complaints can rapidly degrade your domain's reputation, leading to lower inbox placement and, consequently, lower open rates.
Key takeaways from Yahoo's policy updates
Domain reputation: Yahoo prioritizes domain reputation over IP. Ensure your domain's sending history is clean.
Authentication: SPF, DKIM, and DMARC are non-negotiable for high-volume senders. Implement a DMARC policy of p=quarantine or p=reject.
Engagement metrics: Low engagement or high complaint rates will hurt your standing.
What TSS04 and TSS05 errors mean for senders
When you see your open rates plummet, the first thing to check are bounce messages. Yahoo often provides specific error codes that offer clues about the problem. The two most common in recent times have been TSS04 and TSS05.
TSS04: temporary deferral
The TSS04 error code (often appearing as 421 4.7.0 [TSS04] Messages from IP temporarily deferred) indicates that messages are being temporarily deferred. This means Yahoo's servers are not accepting your emails at that moment, but they might accept them later. While seemingly temporary, persistent TSS04 errors can severely impact deliverability and lead to significant drops in open rates because messages are delayed or never reach the inbox effectively.
Common reasons for TSS04 include sudden spikes in email volume from a new or previously low-volume IP or domain, high user complaints (even if overall volume is low), unusual sending patterns, or general spam characteristics detected in your mail stream. For new senders or those warming up a new IP, encountering TSS04 (or similar errors like TSS06) means you're sending too aggressively for Yahoo's liking.
TSS05: content or policy violation
The TSS05 error is more severe. It indicates that your email was not accepted due to something in the content or message characteristics that violates Yahoo's policies. This could range from specific keywords, suspicious links, attachments, or even the overall structure of the email. Unlike TSS04, this is not a temporary deferral, but a hard block.
TSS05 also applies to policy violations, such as poor sender reputation. If you frequently trigger spam traps, have high complaint rates, or fail authentication checks, Yahoo might issue TSS05 errors. Addressing these requires a deeper dive into your email practices.
Error Code
Meaning
Common Causes
Impact on Deliverability
TSS04
Temporary deferral. Yahoo is temporarily not accepting emails.
Unexpected volume spikes, unusual traffic patterns, high bounce rates, or general spam characteristics.
Messages may eventually be delivered, but delays lead to lower open rates and potentially missed engagement opportunities. Can escalate to blocking.
TSS05
Content/policy violation. Email rejected due to content or sender policy.
Emails are blocked and will not be delivered. Requires immediate action to resolve underlying issues.
Key factors affecting Yahoo deliverability
Beyond the specific TSS errors, several factors contribute to declining open rates and deliverability challenges with Yahoo (and generally across all mailbox providers). It's crucial to consider these when troubleshooting why your emails aren't reaching their intended audience.
Authentication failures
As mentioned, proper email authentication is no longer optional. If your SPF, DKIM, or DMARC records are misconfigured or fail alignment checks, your emails are highly likely to be marked as spam or rejected outright. This is especially true with Yahoo's renewed focus on domain reputation. Even if your emails don't bounce, they might land in the spam folder, leading to effectively zero open rates.
Low engagement and high complaint rates
Mailbox providers heavily weigh how recipients interact with your emails. If users aren't opening, clicking, or are consistently marking your emails as spam, it signals to Yahoo that your content is unwanted. This negative feedback can quickly degrade your sender reputation, pushing future emails into the junk folder. This is a primary reason why Yahoo and AOL open rates are dropping.
Content quality and spam triggers
The content of your emails plays a significant role. Generic subject lines, excessive use of exclamation points, all-caps text, or suspicious links can trigger spam filters. Even if your list is opted-in, poor content can lead to emails being filtered or blocked (potentially resulting in TSS05 errors). It's essential to continually review your email content for potential spam triggers.
Common problems leading to drops
Missing DMARC policy: No DMARC record or a policy of p=none leaves your domain vulnerable.
Low open rates: Recipient inactivity signals disinterest to mailbox providers.
High bounce rates: Sending to invalid or old addresses hurts your sender score.
Spam complaints: Even a few complaints can significantly damage your reputation.
Effective solutions to improve deliverability
Implement DMARC: Ensure your DMARC record is set to enforce at least p=quarantine.
Segment inactive users: Remove unengaged subscribers or send them re-engagement campaigns.
Clean your lists regularly: Use suppression lists and validation services to remove invalid addresses.
Monitor feedback loops: Address complaints promptly and remove recipients who complain.
Strategies to restore Yahoo deliverability
While Yahoo's recent changes might seem daunting, there are actionable strategies you can implement to improve your deliverability and mitigate the impact of TSS04 and TSS05 errors.
Strengthen your authentication
Ensure your SPF and DKIM records are correctly configured and aligned with your sending domain. Most critically, implement a DMARC policy with at least p=quarantine. This tells Yahoo how to handle emails that fail authentication. Without DMARC, your legitimate emails might be treated as suspicious.
Maintain a healthy sending reputation
Focus on sending only to engaged subscribers. Regularly clean your email lists by removing inactive users, hard bounces, and those who have unsubscribed. Lowering your complaint rate is paramount, as even a small number of complaints can trigger a blocklist or blacklist issue. Consider re-engagement campaigns for segments of your list to improve overall activity and reduce the number of recipients who might mark your emails as spam.
Optimize content and sending practices
Review your email content for anything that might appear spammy. Avoid excessive images, large attachments, or suspicious links. If you are sending transactional emails, ensure they are clearly identifiable and relevant to the recipient. For new sending IPs or domains, warm up your sending volume gradually to build a positive reputation with Yahoo and avoid triggering TSS04 due to unexpected volume.
Views from the trenches
Best practices
Actively monitor your bounce logs for specific error codes like TSS04 and TSS05 to quickly diagnose issues.
Segment your email lists to send targeted content only to engaged subscribers, improving overall open rates.
Regularly clean your email lists to remove inactive users and invalid addresses, reducing bounce rates.
Implement a strong DMARC policy (p=quarantine or p=reject) to protect your domain's reputation.
Gradually increase sending volume for new IPs or domains to build a positive sending history with mailbox providers.
Common pitfalls
Ignoring temporary deferrals (TSS04) can lead to escalated issues and permanent blocks (TSS05).
Failing to authenticate emails with SPF, DKIM, and DMARC, which is now a strict requirement for high-volume senders.
Not monitoring spam complaint rates, as even a small number can severely damage your sender reputation.
Sending to unengaged or old email addresses, which increases bounce rates and spam trap hits.
Using generic or spammy content that triggers filters, even with a good sending reputation.
Expert tips
Ensure your DNS records for authentication are correctly configured, including SPF and DKIM, and aligned with your sending domains.
Focus on content quality and relevance to foster recipient engagement and reduce spam complaints.
Leverage feedback loops from mailbox providers to automatically remove recipients who mark your emails as spam.
Regularly check your domain and IP against common blacklists (or blocklists) to ensure you're not listed.
Consider segmenting high-volume transactional emails from marketing emails to manage reputation risks more effectively.
Marketer view
A marketer from Email Geeks says they saw a global drop in open rates and tested inboxing across multiple platforms and senders, indicating a widespread issue.
2021-01-29 - Email Geeks
Marketer view
A marketer from Email Geeks says they noticed an uptick in TSS04 errors for a large sender who usually doesn't experience them.
2021-01-29 - Email Geeks
Navigating Yahoo's evolving email landscape
The recent drops in Yahoo open rates and the increase in TSS04 and TSS05 errors highlight a critical shift in how major mailbox providers assess and filter incoming mail. The emphasis on domain reputation, robust authentication, and consistent recipient engagement means that old sending practices may no longer suffice. Email deliverability is an ongoing effort, not a set-it-and-forget-it task.
By understanding the nuances of these error codes and proactively addressing your authentication, list hygiene, and content quality, you can navigate these challenges effectively. Staying vigilant and adapting your strategies will be key to ensuring your emails continue to reach the inbox and achieve the open rates you expect.
