Enhanced SMTP status code · RFC 3463 · IANA
5.7.510 Access denied, [contoso.com] does not accept email over IPv6
The recipient's domain does not accept mail over IPv6.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
- Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
How 5.7.510 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Microsoft 365 / Exchange Online
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5.7.510
Access denied, [contoso.com] does not accept email over IPv6
The recipient's domain does not accept mail over IPv6.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026