How to delist a blacklisted IPv6 address from Office365

Question

I am self hosting an email server, and mail to office365 customers is being blocked with the message:

550 5.7.1 Service unavailable, Client host [2001:41d0:xxxxx] blocked using Spamhaus.

I have checked Spamhaus, and our address is NOT blacklisted there.

I was advised to submit the address via:

https://sender.office.com/

But this site does not support IPv6, despite the fact that Office365 now does by default. How to solve this?

Answer 1

Hello Daniel Smith, 

Since the sender.office.com portal does not currently support IPv6 input, we need to gather a bit more context to find the best alternative route for you.

Could you please provide some additional insights:

1. Have you contacted your ISP or Hosting Provider regarding this? Since you are self-hosting, the provider who owns the IP range (e.g., AWS, DigitalOcean, OVH) often has direct escalation channels with Microsoft to handle false-positive blocks on their subnets.

2. Are your IPv6-specific DNS records configured? Have you verified that your PTR (Reverse DNS) and SPF records explicitly include your IPv6 address? Sometimes a missing IPv6 configuration triggers the filter, even if the error message blames Spamhaus.

3. Is your mail server sending outbound traffic only via IPv6, or is IPv4 also configured? If you have an IPv4 address, have you tried forcing the email to send via the IPv4 outbound path to check if it gets delivered successfully to Exchange Online? This would help isolate whether the issue is specific to the IPv6 route or the domain itself.

The initial answer above may not solve the issue immediately; however, if you could provide this information, it would give us more insight to recommend the appropriate troubleshooting steps.

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