Microsoft (Outlook.com and Microsoft 365) · permanent error
5.7.1: sending IP on the Microsoft 365 blocklist
Microsoft 365 refused the message because the sending IP address is on its own blocklist, named Blocklist 1 in the bounce.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.7.1
- 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 the server says
5.7.1 Service unavailable; Client host … blocked using Blocklist 1
What it means
Microsoft 365 keeps its own list of IP addresses it will not accept mail from. When the sending IP is on it, the sender's bounce carries this 5.7.1 text, naming Blocklist 1 rather than a public list. The text asks you to forward the message to delist@microsoft.com to request removal, and Microsoft's 5.7.1 guidance also points to the delist portal at sender.office.com. Either way, removal only lasts if the traffic that caused the listing has stopped.
Common causes
- The sending IP sent spam or other unwanted mail that Microsoft detected.
- The IP is shared and another sender on it caused the listing.
- A compromised account or server on your network sent from the IP.
How to fix it
- 01Find and stop the source of the unwanted traffic before asking for removal.
- 02Forward the bounce to delist@microsoft.com as it instructs, or request removal at sender.office.com.
- 03Check the IP on public blacklists too, since a listing there often accompanies this one.
- 04Once delisted, keep volume and complaint rates steady to avoid being listed again.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 550 5.7.606–649Banned sending IP: use the delist portal.Microsoft 365 put your sending IP on its blocked senders list; you can request removal yourself at sender.office.com.
- 550 5.7.511Banned sending IP: request removal by email.Microsoft 365 banned the IP you send from and wants to investigate before lifting it; the delist portal cannot remove this block.
- 5.7.1IP listed on both a connector and the IP Allow list.An outside server's IP is set on both an inbound connector and the IP Allow list, which hides the real blocklist that refused it.
5.7.1 at other providers
- 550 5.7.1Blocked by a domain policy.A rule set by the recipient's or the sender's Google Workspace admin blocks this message, so Gmail rejected it under that policy.
- 550 5.7.1Relay IP does not match the sending domain.Google's SMTP relay knows your IP, but the mail's sending domain is not one that IP is registered to relay for in your Workspace account.
- 550 5.7.1Blocked as likely spam.Gmail's filters judged this message likely to be spam and rejected it outright, instead of delivering it to the spam folder.
- 550 5.7.1IPv6 sender without PTR or authentication.Gmail rejected mail that arrived over IPv6 because the sending address lacks a matching PTR record, or the message failed authentication.
- 550 5.7.1Blocked for low IP reputation.Gmail permanently rejected the message because the IP it came from has a very low reputation with Gmail.
- 550 5.7.1Blocked for low domain reputation.Gmail permanently rejected the message because the domain it comes from has a very low reputation with Gmail.
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce