5.7.510: recipient domain does not accept IPv6 mail
You sent over IPv6 to a Microsoft 365 domain whose tenant has not opted in to receiving anonymous mail over IPv6.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.7.510
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
5.7.510 Access denied, [contoso.com] does not accept email over IPv6
What it means
Microsoft 365 can receive anonymous inbound mail over IPv6, but a tenant has to opt in through a support request. Until it does, mail arriving over IPv6 for its domains is refused. Microsoft's error table shows this as 5.7.510 with Access denied wording, and its mail flow documentation shows 550 5.2.1 Service unavailable with the same does not accept email over IPv6 phrase. A sender can get through by delivering over IPv4. After a tenant opts in, IPv6 senders need a PTR record and an SPF or DKIM pass.
Common causes
- Your server connected to Microsoft 365 over IPv6.
- The recipient's tenant has not opted in to anonymous inbound mail over IPv6.
How to fix it
- 01Configure your server to deliver to Microsoft 365 over IPv4.
- 02The recipient's admin can open a support request with Microsoft to enable inbound mail over IPv6.
- 03Before sending over IPv6, give your IPv6 address a PTR record and make sure your mail passes SPF or DKIM.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
- Mail flow in cloud organizations (support for anonymous inbound email over IPv6)learn.microsoft.com
Related codes
- 450 4.7.26IPv6 mail without an SPF or DKIM pass.Microsoft 365 refused a message that arrived over IPv6 because the sender passed neither SPF nor DKIM.
- 5.7.25IPv6 sender without reverse DNS.Microsoft 365 refused mail from an IPv6 address that has no reverse DNS (PTR) record; the same rule also appears as 450 4.7.25.
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