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Microsoft (Outlook.com and Microsoft 365) · permanent error

550 [OU-001]: outlook.com block linked to Spamhaus

Outlook.com rejected the mail for policy reasons and points you to Spamhaus for details of the block and for removal.

Microsoft
Provider
Permanent (5xx)
Class
Sources checked
By , email deliverability consultant
Author

At a glance

Code
550 [OU-001]
Temporary or permanent
PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
Who has to act
The sender. The fix is on the sending side: your domain, your server or your list.

What it means

OU-001 is the Outlook.com code whose explanation sends you to Spamhaus for more information about the block and to request removal, which suggests the block comes from a Spamhaus listing of the IP. Look the IP up on the Spamhaus site, fix whatever caused the listing, and follow the removal steps for the list involved, then try Outlook.com again.

Common causes

  • The IP is listed by Spamhaus.
  • Spam, compromised machines or poor list practices caused the listing.

How to fix it

  1. 01Look up the IP on the Spamhaus website and see which list it is on.
  2. 02Fix the cause described there, then follow the Spamhaus removal steps.
  3. 03Retry Outlook.com after the listing is gone, and contact Outlook.com support if the block remains.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

  1. 550 [DY-001]Outlook.com refuses dynamic IP addresses.Outlook.com rejected the mail because the sending IP is dynamic address space, which it generally does not accept mail from.MicrosoftPermanent

550 at other providers

  1. 550Refused for a header or address problem.Yahoo or AOL refused the message for a policy reason tied to its headers, its addresses or the sending domain's DNS. Yahoo lists these as other policy errors, sent as 550 or 554.YahooPermanent
  2. 550Not an Xfinity customer.The address you sent to does not belong to an Xfinity customer. The failure is permanent, and Xfinity asks bulk mailers to treat it as an unsubscribe.ComcastPermanent
  3. 550Too many invalid recipients.Xfinity refused the whole message because too many of its recipients were invalid, so nobody received it. Clean the recipient list and send it again.ComcastPermanent
  4. 550Sender domain has no A or MX record.Xfinity refused the message because the domain you send from has neither an A record nor an MX record in DNS. Add one and send again.ComcastPermanent
  5. 550Account not available.The Xfinity account you wrote to is currently not available. Xfinity sends this as a 550, so the message has failed, though the account itself may come back.ComcastPermanent
  6. 550Permanent reject under an exceptional blocklist.GMX permanently refused mail from your system because it is on one of the blocklists GMX uses in exceptional situations. The message comes back and is not retried.GMX, WEB.DEPermanent

Further reading

All 111 Microsoft codesEvery code in the wikiPaste your own bounce

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