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

550 [SC-003]: outlook.com block for an open proxy or relay

Outlook.com rejected the mail because the sending IP appears to be an open proxy or open relay.

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

At a glance

Code
550 [SC-003]
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

Outlook.com refuses mail from IP addresses that appear to be an open proxy or relay, a system that passes on mail or traffic for anyone. Its policies also say messages must not be sent through insecure relay or proxy servers. If your IP gets SC-003, check whether your mail server relays for unauthenticated clients, or whether a proxy service on the same IP is reachable from the internet. Once it is closed, request a review.

Common causes

  • The mail server relays mail for anyone without authentication.
  • A proxy service on the IP is open to the internet.
  • Your mail passes through a third-party relay that is open.

How to fix it

  1. 01Test your mail server for open relay and require authentication for relaying.
  2. 02Close or restrict any open proxy services on the IP.
  3. 03After fixing it, submit the Outlook.com sender support form for the IP.

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-002]Outlook.com block for a likely compromised sender.Outlook.com rejected the mail because the sending server or computer appears to be compromised or infected.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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