# 550 5.7.1 Service unavailable; client host blocked using a blocklist

URL: https://iblockspam.com/smtp-errors/550-5-7-1-client-host-blocked
Last updated: 2026-10-09

Code: 550 5.7.1 (permanent failure, the message bounced)
Provider: Microsoft 365 / Outlook.com

Microsoft 365 refused mail from your sending IP because the IP is on a blocklist. The text names the list and how to request removal.

What the bounce says:

```
5.7.1 Service unavailable; Client host [xxx.xxx.xxx.xxx] blocked using Blocklist 1; To request removal from this list please forward this message to delist@microsoft.com
```
Microsoft's example from its 550 5.7.1 article, for a sender whose IP is on Microsoft's blocklist.

Microsoft's guide for 550 5.7.1 covers the case where the sender is external and their source IP address is on Microsoft's blocklist. The NDR's diagnostic section then contains text like the example above, and Microsoft's fix is to forward the NDR message to delist@microsoft.com, or use its delist portal.

The same line format, "Client host [IP] blocked using ...", names whichever list caused the block, and senders also report versions that name a public blocklist such as Spamhaus instead. When a public list is named, Microsoft's delist process doesn't apply: look the IP up on that list's own website and follow its removal steps. Yahoo, for example, tells senders whose IP is listed by Spamhaus to check with Spamhaus.

Either way, removal only sticks once you know why the IP was listed. Typical reasons are spam from a compromised account or website, a misconfigured server sending to many invalid addresses, or a dynamic or residential IP that shouldn't send mail directly.

## Common causes

- Your sending IP is on Microsoft's own blocklist.
- Your sending IP is on a public DNS blocklist that the receiver checks.
- The IP sent spam, often from a compromised account, server or website.
- The IP is a dynamic or residential address that lists flag as not meant to send mail.

## How to fix it (sender)

- Read which list the error names.
- For Microsoft's list: forward the NDR to delist@microsoft.com or use the delist portal at sender.office.com.
- For a public list: look the IP up on that list's site, fix the cause, then request removal.
- Check the IP against other public blocklists while you're at it.

## How to fix it (recipient or admin)

- If a wanted partner is blocked by your own organization's settings, review your connection filtering and allow lists.

**Q: The error names Spamhaus. Can Microsoft remove me?**
No. When the error names a public blocklist, removal goes through that list's own process. Look up your IP on the list's website, fix the cause it describes, and request removal there.

**Q: Why can't your Blacklist Checker test Spamhaus?**
Spamhaus refuses lookups that come through large public DNS resolvers, so our checker shows it as "not tested" with a link rather than reporting a result we couldn't get. Use Spamhaus's own lookup page.

Sources:
- Fix NDR error "550 5.7.1" in Exchange Online (Microsoft Learn): https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/fix-error-code-550-5-7-1-in-exchange-online
- Remove yourself from the blocked senders list and address 5.7.511 Access denied errors (Microsoft Learn): https://learn.microsoft.com/en-us/defender-office-365/external-senders-use-the-delist-portal-to-unblock-yourself
- SMTP Error Codes (Yahoo Sender Hub): https://senders.yahooinc.com/smtp-error-codes/

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