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

550 [SC-002]: outlook.com block for namespace mining

Outlook.com blocked the sending IP because it behaved like a namespace miner, checking whether addresses exist without sending mail.

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

At a glance

Code
550 [SC-002]
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

Namespace mining means verifying email addresses against a mail server without sending, or attempting to send, a message to them. Spammers use it to build lists, and Outlook.com does not allow it: it takes action against IPs that do it and rejects their mail with SC-002. Legitimate senders get caught when an address verification tool or script probes Outlook.com from their IP, or when an attacker uses a compromised server to harvest addresses.

Common causes

  • An address verification service or script checked Outlook.com addresses from your IP without sending.
  • A compromised machine or account on your network is harvesting addresses.

How to fix it

  1. 01Stop any address checking against Outlook.com servers that does not send mail.
  2. 02Check your servers and sending accounts for compromise.
  3. 03Validate lists through confirmed opt-in and bounce handling instead of SMTP probing.
  4. 04Once the behavior has stopped, submit the Outlook.com sender support form to request a review.

Sources

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

  1. 550 [SC-001]Outlook.com policy block for content or reputation.Outlook.com rejected the mail for policy reasons tied to spam-like content or to the reputation of the sending IP or domain.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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