Fix Microsoft 365 Emails Going to Yahoo Junk (CYMRU BOGONS IPv6 Issue)
Question details
Users experience Microsoft 365 emails being routed to the Yahoo Junk folder because a Microsoft relay IP address appears on the CYMRU BOGONS IPv6 blacklist, despite passing SPF, DKIM, and DMARC.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Sending legitimate business emails from Microsoft 365 / Exchange to Yahoo Mail recipients.
- Observed behavior
- Messages are delivered to the recipient's Junk folder. Email headers indicate that a Microsoft IPv6 relay server is listed on the CYMRU BOGONS blacklist, which is often a false positive masking underlying SPF record misconfigurations.
Before proceeding, ensure you have administrative access to your domain registrar's DNS management portal and can run a message trace in the Microsoft 365 Exchange admin center.
Review and Update Your Domain's SPF Record
The CYMRU BOGONS IPv6 listing is often a false positive. The most common real cause for emails going to junk in this scenario is an overly permissive SPF record ending in '+all'.
Blacklist checkers sometimes flag internal Microsoft IPv6 servers incorrectly, causing a 'red herring' during troubleshooting. Spam filters at Yahoo may actually be penalizing your emails because your SPF record uses '+all', which technically allows any server on the internet to send email on your behalf.
Log in to your domain registrar's DNS management dashboard (e.g., GoDaddy, Cloudflare, Namecheap).
Navigate to the DNS records section and find your existing domain TXT records. Look for the record that begins with 'v=spf1'.
Check the end of the SPF record. If it ends with '+all', edit the record and change the suffix to '~all' (soft fail) or '-all' (hard fail).
Ensure all authorized third-party senders and Microsoft 365 ('include:spf.protection.outlook.com') are explicitly listed in the record before saving.
Click 'Save' to apply the DNS changes. Wait up to 48 hours for the new DNS records to propagate globally.
Run a Message Trace and Escalate to Microsoft Support
If your DNS records are correctly configured and the issue persists, you may need Microsoft to investigate the specific relay IP's routing reputation.
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Frequently Asked Questions
Why is a Microsoft 365 IP listed on the CYMRU BOGONS IPv6 blacklist?
Blacklist checkers like MXToolbox can sometimes mistakenly flag internal Microsoft IPv6 relay servers as bogons. In many cases, this is a false positive (a "red herring") and not the actual reason your emails are being routed to the junk folder.
Will changing my SPF record to '-all' fix email deliverability to Yahoo?
If your current SPF record ends with '+all', it allows any server on the internet to send emails on behalf of your domain, which immediately triggers spam filters. Updating the suffix to '-all' (hard fail) or '~all' (soft fail) secures your domain and frequently resolves deliverability issues with providers like Yahoo.
Can I manually remove the Microsoft relay IP from the blacklist?
No, end users and domain administrators cannot directly request the removal of internal Microsoft relay IPs from third-party blacklists. If the IP reputation truly needs to be corrected, you must escalate the issue to Microsoft Support for investigation.
