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550 5.7.510: Recipient doesn't accept email over IPv6 (Microsoft)

You delivered over IPv6 to a Microsoft 365 domain that only accepts mail over IPv4.

Bounce type
Hard bounce (permanent)
What to do
Fix your setup, keep the contact
Returned by
Microsoft (Outlook.com, Microsoft 365)

What 550 5.7.510 means

Microsoft documents 5.7.510 as 'Access denied, [domain] does not accept email over IPv6'. Your server connected to Microsoft 365 over IPv6, but the recipient organization hasn't enabled inbound IPv6, so the message was refused.

Nothing is wrong with the address or with your reputation. It's a transport mismatch that only affects self-managed servers that prefer IPv6.

What the bounce looks like

The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.

Microsoft
550 5.7.510 Access denied, [domain] does not accept email over IPv6

Why cold emailers see 550 5.7.510

  • A self-hosted MTA (Postfix, Exim) with IPv6 enabled that prefers IPv6 when the MX has an AAAA record.
  • An SMTP provider routing some traffic over IPv6.
  • A new cloud server where IPv6 is on by default, so the MTA starts using it without anyone choosing to.

How to fix 550 5.7.510

  1. Configure your MTA to deliver over IPv4 for these destinations, or entirely (in Postfix, smtp_address_preference = ipv4 or inet_protocols = ipv4).
  2. If you need IPv6, make sure your sending IPv6 address has reverse DNS and passes SPF or DKIM, which Microsoft requires for IPv6 senders anyway.
  3. Resend after the change.

Retry or suppress?

Don't suppress. Switch delivery to IPv4 and resend.

Reading the code

The basic code 550 means "Mailbox unavailable or message refused (permanent)" under RFC 5321. In 5.7.510, the 5 is the class (permanent failure) and the 7 is the subject (security or policy). The detail number 510 isn't in the RFC 3463 / IANA registry; it's specific to the provider, so the text after the code carries the meaning.

Class 5
Permanent failure
Resending the same message won't work. Something about the message, the recipient or the sender has to change first.
Subject 7
Security or policy
A security or policy decision: spam filtering, authentication, blocklists, permissions.
Detail 5.7.510
Provider-specific

Check the rest of your setup

Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.

Got a different bounce? Paste it into the SMTP error code lookup to decode it.

FAQ

Is 550 5.7.510 a hard bounce or a soft bounce?

550 5.7.510 is a permanent failure (a hard bounce): the receiving server won't accept this message if you send it again unchanged. But the cause is not the recipient's address, so treat it differently from a 'user unknown' bounce.

Should I remove the contact after a 550 5.7.510 bounce?

No. Don't suppress. Switch delivery to IPv4 and resend.

What does 5.7.510 mean in the error code?

In 5.7.510, the 5 is the class (permanent failure) and the 7 is the subject (security or policy). The detail number 510 isn't in the RFC 3463 / IANA registry; it's specific to the provider, so the text after the code carries the meaning.

Related SMTP codes

See all SMTP error codes

Sources

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