If you are seeing deliverability issues at Yahoo or AOL, such as emails being rejected, delayed, or landing in spam, this article covers the most common causes and how to fix them.
Before you start
- Find your bounce details in Brevo. Bounce details include the error code returned by Yahoo's mail servers. To learn how to find them, check our dedicated article What are soft bounces and hard bounces in email?.
- Enroll in Yahoo's Complaint Feedback Loop. The Complaint Feedback Loop, or CFL, is Yahoo's free tool for seeing which of your emails get marked as spam. To set it up, follow Yahoo's official documentation.
Find the right section
Use the tabs below to navigate to the section that matches your situation.
Use this table to match your symptom to its likely cause and find the right section.
| What you are seeing | Possible cause | Go to |
|---|---|---|
| Emails rejected outright, referencing your sending domain or its authentication | Domain authentication failure | Domain authentication failure |
| Emails rejected or filtered, with no clear technical cause | A high spam complaint rate | High complaint rate |
| Emails delayed but eventually delivered | Temporary rate limiting after a volume increase or due to your sender reputation | Delivery delays and rate limiting |
| Emails bounce referencing a header or formatting problem | Non-standard message headers | RFC compliance and malformed headers |
| Emails rejected as suspicious or unsafe | Content flagged as phishing, malware, or otherwise unsafe | Blocked for malicious content |
| The bounce message names a specific blocklist, such as Spamhaus | Your IP is listed on an external reputation blocklist that Yahoo checks | Blocked by an external blocklist |
| You cannot tell which contacts are marking your emails as spam | You are not enrolled in Yahoo's Complaint Feedback Loop | No visibility into complaints |
Yahoo groups its bounce codes into a few broad categories rather than one code per issue. Always read the full bounce message text alongside the code.
| Error code | Possible cause | Go to |
|---|---|---|
5xx, for example 553 or 554, referencing authentication |
Your sending domain failed an SPF, DKIM, or DMARC check | Domain authentication failure |
| Message referencing excessive user complaints | Your spam complaint rate is above Yahoo's threshold | High complaint rate |
4xx, for example 421 or 451, or a TS* code such as TSS04
|
Temporary deferral, often due to sudden volume or borderline reputation | Delivery delays and rate limiting |
| Message referencing RFC compliance, or an unresolvable domain | Malformed or non-standard message headers | RFC compliance and malformed headers |
| A PH* code, or a message referencing objectionable content | Your content was flagged as phishing, malware, or otherwise unsafe | Blocked for malicious content |
Domain authentication failure
Domain authentication is how Yahoo checks that your email was genuinely sent by you, and not by someone pretending to be you. Yahoo requires bulk senders to pass DKIM (and SPF for dedicated IP users), with the domain in your "From" address aligned to one of them for DMARC. When authentication fails, Yahoo rejects the email outright.
Signs of domain authentication failure
- Bounce error codes in the
553or554range, with a message referencing authentication, DKIM, or DMARC. - Most or all of your emails to Yahoo or AOL addresses are failing, not just one campaign.
Why this happens
- DKIM (or SPF for dedicated IP users) is missing, outdated, or does not include Brevo's sending servers.
- The domain in your "From" address does not match the domain your DKIM record is set up for.
- You are sending from multiple platforms with conflicting DNS records.
How to check your domain authentication
To check your domain authentication, review your Authentication-Results header in Yahoo Mail:
- In Yahoo Mail, open a delivered email sent from your Brevo account.
- Click More and select View Raw Message.
- Find the Authentication-Results header.
- Confirm that you see dkim=pass, spf=pass, and dmarc=pass.
How to fix domain authentication issues
➡️ To learn how to fix domain authentication issues, check our dedicated article Troubleshooting - Domain authentication.
High complaint rate
Yahoo requires bulk senders to keep their spam complaint rate below 0.3%. In practice, staying well under this threshold, closer to 0.1%, gives you a safer margin.
Signs of a high complaint rate
- A bounce message referencing excessive user complaints.
- Emails increasingly landing in the Spam folder rather than being rejected outright.
- Delivery declining specifically for Yahoo and AOL contacts while other providers remain stable.
Why this happens
- You are sending to old, unengaged, or purchased contact lists.
- Recipients cannot find or use your unsubscribe link easily, so they mark your email as spam instead.
- Your email content or sending pattern does not match what recipients originally signed up for.
What to do about a high complaint rate
- Send only to contacts who have opened or clicked one of your emails recently, and move unengaged contacts to a separate list.
- Make sure your unsubscribe link is visible and works without requiring a login.
- Keep your sending frequency consistent with what contacts agreed to when they signed up.
➡️ For more guidance on rebuilding your sender reputation, check our dedicated articles Best practices for email deliverability and Optimize and clean your contact list to improve email deliverability.
Delivery delays and rate limiting
If you send a large number of emails too quickly, or use a new sending IP or domain, Yahoo temporarily delays delivery. These are temporary holds, not permanent blocks. Emails are retried automatically, so there is no need to resend them manually.
Signs of rate limiting
- Bounce error codes in the
421or451range, or a code starting with TS, such asTSS04. - A large send is partially delivered, with a significant percentage deferred.
Why this happens
- You sent a significantly larger volume than on previous days without gradually warming up.
- You are using a new IP or domain with no established sending history.
- Your recent complaint rate or content is borderline, so Yahoo is throttling proactively.
What to do to avoid rate limiting
- Spread large sends across multiple hours or days rather than sending everything at once.
- If you are using a new dedicated IP, warm it up first. To learn more, check our dedicated article Warm up your dedicated IP (manual warm-up) or Warm up your dedicated IP (automatic warm-up).
RFC compliance and malformed headers
Every email carries hidden technical details called headers, covering things like who sent it and how it is formatted. These must follow a strict structure defined by two standards:
- RFC 5321, which governs how the message is transmitted, and
- RFC 5322, which governs how the message itself is formatted.
If either is not followed, Yahoo rejects the email regardless of its content.
Signs of a compliance failure
- A bounce message referencing a formatting or compliance problem with your email, rather than its content or your reputation.
- The issue is consistent and not tied to a specific campaign.
Why this happens
These issues almost always come from a custom integration rather than Brevo's own sending process. Common causes include:
- Duplicate headers (e.g., two "Subject" or "From" lines in the same email).
- Incorrectly formatted headers or an incorrect MIME type, which tells the receiving server what kind of content the email contains.
- A sending domain missing the DNS record needed to confirm it exists.
How to fix a compliance failure
Review how your integration formats outgoing emails against RFC 5321 and RFC 5322. If the issue continues, check our dedicated article Troubleshooting issues with Brevo SMTP.
Blocked for malicious content
Yahoo rejects emails it identifies as containing links or attachments associated with phishing, malware, or ransomware.
Signs of a content-based block
A bounce message with a code starting with PH, or referencing objectionable content.
How to fix a content-based block
- Review your email for links to domains you do not control or recognize, since a compromised or malicious linked domain can trigger this block.
- Remove any attachments that are not essential to the email.
- If you believe the block is a false positive, submit a Sender Support Request with the specific error and diagnostic details from your logs.
Blocked by an external blocklist
Yahoo's rejection message sometimes names a specific third-party reputation blocklist, such as Spamhaus. This means your IP has been listed by that blocklist provider, not by Yahoo itself.
Signs of an external blocklist reference
Bounce error codes in the 553 or 554 range, with the message naming a specific blocklist provider.
How to fix an external blocklist reference
➡️ To fix an external blocklist reference, check our dedicated article FAQs - Why is my IP address or domain blocklisted?.
No visibility into complaints
The Complaint Feedback Loop, or CFL, is Yahoo's mechanism for sending complaint reports back to senders. When a Yahoo or AOL recipient marks your email as spam, CFL sends a report to your registered address. Without it, these complaints stay invisible until your reputation has already been affected.
How to enroll in the CFL
- Go to Yahoo's Complaint Feedback Loop page and select Enroll Now.
- Create your sender profile.
- Add and verify the DKIM-signed domain you want to enroll.
- Enroll the domain in the CFL.
Diagnostic tools
The following tools can help you diagnose and monitor deliverability to Yahoo and AOL addresses:
| Tool | URL | Who can use it | What you can do |
|---|---|---|---|
| Complaint Feedback Loop | senders.yahooinc.com | All senders with a DKIM-signed domain | Receive reports when recipients mark your email as spam |
| Sender Hub error code reference | senders.yahooinc.com | All senders | Look up the meaning of a specific bounce code |
| Sender Support Request | senders.yahooinc.com | All senders | Escalate a persistent delivery problem to Yahoo directly |
| Yahoo Mail "View Raw Message" | In Yahoo Mail, open the email, click More, then View Raw Message | All senders | View the raw message headers to confirm authentication results |
⏭️ What's next?
- Comply with Gmail, Yahoo, and Microsoft's requirements for email senders
- Troubleshooting - Domain authentication
- FAQs - Why is my IP address or domain blocklisted?
- Best practices for email deliverability
- Best practices for managing a dedicated IP
- Troubleshooting - Deliverability issues with Gmail
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