550 5.7.510: Microsoft 365 refuses mail over IPv6
550 5.7.510Access denied, recipient does not accept email over IPv6Permanent: retrying will not helpFrom Microsoft 365 and Outlook.com
550 5.7.510 Access denied, example.com does not accept email over IPv6
What it means
Exchange Online can receive over IPv6 only for tenants that turned it on. When your server connects to a Microsoft 365 recipient over IPv6 and that domain has not enabled IPv6 inbound, the message is refused with 5.7.510, and the text typically names the recipient domain.
This is not a reputation problem and does not mean your IPv6 address is blocked. It is the recipient’s configuration. The nearest neighbour is 5.7.508, which is a volume limit on IPv6 senders, while 5.7.510 is a plain "not over this protocol".
Either side can solve it: the sender by delivering over IPv4, or the recipient’s admin by enabling IPv6 for the domain in Exchange Online.
Likely causes
- Your outbound MTA prefers IPv6 and the recipient tenant only accepts IPv4.
- The recipient published AAAA records for its MX through a third party, while the Microsoft side does not accept IPv6 for that domain.
- A recent network change gave your mail server IPv6 connectivity it did not have before.
How to fix it
- As the sender, set your MTA to use IPv4 for outbound SMTP, at least for Microsoft hosts (in Postfix, `smtp_address_preference = ipv4` or `inet_protocols = ipv4`).
- Requeue the bounced messages after the change.
- As the recipient’s admin, decide whether you want IPv6 inbound; if so, enable it for the domain with `Enable-IPv6ForAcceptedDomain` in Exchange Online PowerShell.
- After enabling it, give DNS time to update and test from an IPv6 sender.
Checks that apply
- MX Lookup Lists the recipient’s MX hosts so you can see whether they publish IPv6 addresses at all.
- AAAA Record Shows whether the MX host names have AAAA records, which is what invites your server to try IPv6.
- SMTP Diagnostics Connects to the MX the way a sender would and reports the banner, confirming the host is reachable over IPv4.
FAQ
- Is my IPv6 address blacklisted?
- No. 5.7.510 is about the recipient domain not accepting IPv6, not about your address’s reputation.
- Why does it happen only for some Microsoft 365 recipients?
- IPv6 inbound is a per domain setting, so tenants that enabled it accept your IPv6 mail and others refuse it.