Why are emails being blocked by Yahoo/AOL in February 2022?
Michael Ko
Co-founder & CEO, Suped
Published 28 Jun 2025
Updated 19 Aug 2025
6 min read
In February 2022, many email senders encountered unexpected challenges with email delivery to Yahoo and AOL recipients. Reports of messages being blocked or significantly delayed surged, leading to widespread concern among marketers and businesses relying on email communication. This sudden shift in deliverability prompted an investigation into the underlying causes.
The primary symptoms observed were email deferrals, specifically 421 errors, and in some cases, TSS04 errors. These temporary failures indicated that Yahoo and AOL were actively throttling or rejecting incoming mail, signaling a heightened level of filtering. This was not an isolated incident affecting a few senders, but rather a system-wide phenomenon that impacted many.
Understanding why these email blockages occurred requires looking at the technical signals Yahoo and AOL use to assess incoming mail. Their systems constantly evaluate sender trustworthiness based on various factors. When these factors don't align with their expectations, even legitimate emails can find themselves caught in the filters.
When you see a 421 error, it typically means the recipient's mail server is temporarily unavailable or experiencing a high volume of traffic, leading to a deferred delivery. For Yahoo and AOL, however, it often indicates active throttling due to perceived issues with the sending IP or domain. The server is essentially saying, "Try again later, but we're not too keen on your mail right now."
The TSS04 error is more specific to Yahoo and AOL (Oath/Verizon Media) properties. It suggests a problem with the sender's IP address or domain's reputation, often related to suspicious activity or exceeding set volume limits. Both errors, when seen widely and consistently, point to a deliberate filtering action by the mailbox providers.
These aren't hard blocks, but rather a soft bounce or deferral. However, if emails continue to defer, they can eventually lead to a hard bounce. This pattern in February 2022 suggested that Yahoo and AOL were tightening their filtering mechanisms, potentially in response to increased spam or abuse attempts, or a general adjustment to their deliverability thresholds.
Factors contributing to the blocking incident
Mailbox providers like Yahoo and AOL utilize complex algorithms to determine if an email is legitimate or spam. While the exact trigger for the February 2022 issues isn't publicly disclosed, common factors often play a role in such widespread blocking events. These primarily revolve around sender reputation and email authentication.
Sender reputation is crucial. This includes both your IP reputation and, increasingly, your domain reputation. A sudden influx of complaints, high bounce rates, sending to unknown users, or even being listed on a public email blacklist (or blocklist) can severely impact your standing with mailbox providers. It is important to remember that some blocklists are private and only known to the mailbox providers themselves. If your sending practices suddenly changed or if a third-party service provider had an issue, it could trigger these reputation warnings.
Proper email authentication is another non-negotiable requirement. SPF, DKIM, and DMARC records verify that an email truly originates from the domain it claims to be from. Without these in place, or if they are misconfigured, emails are far more likely to be flagged as suspicious or spam. Yahoo and AOL have historically been strong proponents of robust authentication. You can read more about what a DNSBL is and how it works.
Key authentication protocols
SPF: Sender Policy Framework verifies the sending IP address is authorized by the domain owner. You can learn more about a simple guide to DMARC, SPF, and DKIM.
DKIM: DomainKeys Identified Mail adds a digital signature to your emails, ensuring the message hasn't been tampered with in transit.
DMARC: Domain-based Message Authentication, Reporting, and Conformance builds on SPF and DKIM, telling receiving servers how to handle emails that fail authentication. Read our guide to DMARC tags and their meanings for more information.
Additionally, the content of your emails, list quality, and user engagement play a significant role. If emails contain spammy keywords, suspicious links, or if recipients frequently mark your emails as spam, it can negatively impact your deliverability. Maintaining a clean, engaged list is paramount to avoiding future blockages or blacklists.
Strategies for recovery and prevention
If you experienced blocking by Yahoo or AOL in February 2022, or encounter similar issues now, a systematic approach is necessary for recovery. Start by checking your email logs for the specific error codes. This will confirm if you are indeed facing deferrals from these providers. Next, review your sender reputation metrics, including complaint rates, bounce rates, and any blocklist (or blacklist) listings.
Proactive measures
Monitor delivery: Regularly check your email sending logs and deliverability reports for any unusual spikes in bounces or deferrals, especially to major mailbox providers like Google and Yahoo.
Authenticate rigorously: Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. A valid DMARC policy is critical.
Segment and clean lists: Regularly remove inactive or unengaged subscribers. This reduces bounce rates and spam trap hits. Inactive email addresses can often lead to future email deliverability issues.
Reacting to blockages
Analyze bounce codes: Differentiate between temporary (4xx) and permanent (5xx) failures. The 421 and TSS04 are temporary, but repeated occurrences signal deeper problems.
Review sending practices: Evaluate recent campaign content, list acquisition methods, and sending volume changes that might have triggered the filtering.
Contact postmasters: If problems persist despite internal checks, engage directly with the Yahoo and AOL postmaster teams through their dedicated channels. This is an important way to resolve bouncing and blocking issues.
Implementing a strong DMARC policy is highly recommended. Not only does it protect your brand from email spoofing, but it also provides valuable feedback on how your emails are performing at various mailbox providers. This data is essential for diagnosing and resolving deliverability issues, including understanding why your emails are going to spam.
Staying proactive with your email program's health means continuous monitoring of your domain and IP reputation. Yahoo (and AOL) have become increasingly strict, moving towards a heavy reliance on domain-based signals rather than just IP addresses. This shift means that even if you're using a shared IP, your domain's sending history and authentication practices are paramount. You can also monitor your domain reputation using Google Postmaster Tools.
Views from the trenches
Best practices
Maintain consistent sending volumes and avoid sudden spikes in email campaigns.
Regularly clean your email lists to remove inactive or bouncing addresses.
Implement strong SPF, DKIM, and DMARC authentication for all sending domains.
Monitor feedback loops and promptly address any complaint spikes.
Segment your audience to send targeted, highly engaging content.
Common pitfalls
Ignoring 421 or TSS04 deferrals, assuming they are temporary and will resolve themselves.
Not having DMARC in place or having it set to 'p=none' without proper monitoring.
Using purchased or old lists with high rates of unknown users or spam traps.
Over-relying on IP reputation when domain reputation has become more critical.
Failing to adapt to evolving mailbox provider filtering policies and requirements.
Expert tips
Use a reputable email service provider (ESP) that handles IP warming and maintenance effectively.
Leverage DMARC reports to gain insights into authentication failures and potential abuse.
Ensure your content is relevant and personalized to minimize spam complaints.
Set up an easy-to-find unsubscribe link that works immediately.
Regularly check major blocklists (blacklists) to ensure your sending IPs and domains are clean.
Marketer view
A marketer from Email Geeks says they saw widespread 421 errors starting last night, indicating a sudden and unexplained change in Yahoo/AOL's filtering.
2022-02-25 - Email Geeks
Marketer view
A marketer from Email Geeks says their deferrals were system-wide, starting on the 24th and remaining relentless with no clear reason for the change.
2022-02-26 - Email Geeks
Moving forward with strong deliverability
The February 2022 email blocking incident by Yahoo and AOL served as a stark reminder of the dynamic nature of email deliverability. While specific reasons for such events are often proprietary to the mailbox providers, they almost always boil down to reputation and authentication. Mailbox providers are constantly refining their filters to combat spam and protect their users.
For senders, this means that merely adhering to basic best practices might not be enough. Continuous monitoring, proactive list management, and robust email authentication (SPF, DKIM, and DMARC) are essential. Understanding why your emails fail will empower you to react swiftly to issues like the 421 and TSS04 errors.
By prioritizing sender reputation and diligently managing your email program, you can significantly reduce the risk of your emails being blocked (or blacklisted) by major providers. This not only ensures your messages reach the inbox but also builds trust with your recipients and contributes to a healthier email ecosystem overall.
