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

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.

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

At a glance

Code
550 [DY-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

Outlook.com generally refuses mail from dynamic IP addresses, the kind ISPs hand out to home and office connections, because such addresses are rarely meant to pass mail straight to other mail servers. Its explanation points to Spamhaus, which maintains lists of dynamic and residential IP addresses. Mail sent straight from a home or office connection, rather than through a mail server or provider with a static address, typically meets this block.

Common causes

  • Mail is sent directly from an internet connection with a dynamic IP.
  • The IP is listed by Spamhaus as dynamic or residential space.

How to fix it

  1. 01Send through your provider's or ISP's mail server instead of delivering directly.
  2. 02If the IP is static and should send mail, ask the ISP to give it a proper reverse DNS name and to have it removed from dynamic lists.
  3. 03Check the IP against Spamhaus before resending.

Sources

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

  1. 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.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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