5.7.510: Recipient does not accept email over IPv6
Microsoft 365 returns 5.7.510 when you delivered over IPv6 to a recipient domain that does not accept email over IPv6. Configure your sending server to deliver to that domain over IPv4, or prefer IPv4 for all outbound mail in general.
The exact message
Microsoft 365 / Exchange Online
5.7.510 Access denied, [contoso.com] does not accept email over IPv6
Why it happens
- Your server has an IPv6 address and used it to deliver.
How to fix it
- Prefer IPv4 for outbound SMTP, for example with Postfix’s smtp_address_preference = ipv4.
If you send cold email
Only relevant to self-hosted sending. Mailbox-provider sending handles this automatically.
How WarmHawk handles it
A Google Workspace or Microsoft 365 mailbox connected to WarmHawk sends through the provider’s own servers, which already use TLS. A temporary failure is retried with backoff (30 minutes, then doubling, capped at 48 hours) and the lead is suppressed after 4 attempts instead of being retried forever. How WarmHawk works →
Check your domain now
These free checkers read your live DNS: no account, up to 15 domains at once.
Related bounce codes
Sources, checked 2026-09-29: Microsoft Learn: NDRs and SMTP errors in Exchange Online. Have a different bounce? Paste it into the decoder →
Questions
5.7.510 questions
What does 5.7.510 mean?+
Microsoft 365 returns 5.7.510 when you delivered over IPv6 to a recipient domain that does not accept email over IPv6. Configure your sending server to deliver to that domain over IPv4, or prefer IPv4 for all outbound mail in general.
Is 5.7.510 a temporary or permanent error?+
Permanent. The message will not be retried, and sending it again unchanged will fail the same way until the cause is fixed.
How do I fix 5.7.510?+
Prefer IPv4 for outbound SMTP, for example with Postfix’s smtp_address_preference = ipv4.