# 550 5.7.511: access denied, banned sender

URL: https://iblockspam.com/smtp-errors/550-5-7-511
Last updated: 2026-10-09

Code: 550 5.7.511 (permanent failure, the message bounced)
Provider: Microsoft 365 / Outlook.com

Your sending IP address is banned by Microsoft 365 and needs further investigation. Unlike other blocks, you can't use the self-service delist portal.

What the bounce says:

```
550 5.7.511 Access denied, banned sender[xxx.xxx.xxx.xxx]. To request removal from this list, forward this message to delist@microsoft.com.
```
Start of the example NDR in Microsoft's delist portal article.

Microsoft's NDR reference describes 5.7.511 as "Access denied, banned sender": the IP you're trying to send from was banned. Its delist article explains the difference from the more common 5.7.606 to 5.7.649 blocks: in some cases Microsoft needs to investigate traffic from the IP further, so the self-service delist portal won't work.

To request removal, do what the NDR says: send a message to delist@microsoft.com and include the full NDR code and the IP address. Microsoft says it will contact you within 48 hours with next steps.

Use those 48 hours. A 5.7.511 usually follows serious abuse from the IP, such as a compromised server or account sending large amounts of spam. Have a clear answer about what happened and what you changed, because Microsoft can block the IP again if bad traffic continues.

## Common causes

- Significant spam or malicious traffic from your IP address.
- A compromised server, website or mailbox using your IP.
- A shared IP abused by another sender.

## How to fix it (sender)

- Find and stop the abuse source before contacting Microsoft.
- Email delist@microsoft.com with the full NDR code and the IP address.
- Expect a reply within 48 hours, and be ready to explain what you fixed.

## How to fix it (recipient or admin)

- Recipients can't lift this block; the sender has to work with Microsoft.

**Q: Can I use sender.office.com for 5.7.511?**
No. Microsoft says the delist portal doesn't work for 5.7.511. Forward the NDR to delist@microsoft.com with the full NDR code and IP address instead.

**Q: How long does Microsoft take to respond?**
Microsoft says it will contact you within 48 hours with next steps.

Sources:
- 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
- Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn): https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/non-delivery-reports-in-exchange-online

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Source: iBlockSpam, https://iblockspam.com/smtp-errors/550-5-7-511 (this is a Markdown copy; the HTML page is canonical). Quote with attribution and a link. Site index for AI assistants: https://iblockspam.com/llms.txt