Send & Receive Issues

How to Fix Error 5.7.511 When Sending Email to Microsoft 365 Users

Ayan Masood Oct 7, 2026 868 views

Question details

The user needs to resolve an email delivery failure to Microsoft 365 recipients caused by an SMTP 5.7.511 error.

How to Fix Error 5.7.511 When Sending Email to Microsoft 365 Users

Product: Microsoft 365 / Exchange Online
Device & OS: not provided
Scenario: Sending emails from a third-party server or domain to users hosted on Microsoft 365.
Observed behavior: The email is rejected and returned with an SMTP error code 5.7.511, indicating that Exchange Online Protection has blocked the sender's IP address due to suspected spam or poor reputation.

Before you start

Ensure you have administrator access to your email server and DNS settings. You will need to gather the rejected email's non-delivery report (NDR) to confirm the blocked IP address before submitting a delisting request.

Solution 1Recommended

Request Delisting from Microsoft and Authenticate Email

Resolve the IP block by checking public blocklists, requesting removal from Microsoft's anti-spam portal, and setting up proper email authentication protocols.

SMTP error 5.7.511 is a direct result of Microsoft's Exchange Online Protection (EOP) flagging your mail server's IP address. This typically happens if the IP is found on a global spam list, if there is a sudden spike in outbound email volume, or if proper domain authentication is missing.

To permanently fix this issue, you must not only request removal from the blocklist but also address the underlying vulnerabilities, such as configuring proper DNS records to verify your sending identity.

Check IP Reputation

Use online diagnostic tools like MXToolBox to check if your mail server's IP address is currently listed on any public spam blocklists (RBLs).

Submit a Delisting Request to Microsoft

Navigate to the Office 365 Anti-Spam IP Delist Portal. Enter the email address you are sending from and the blocked IP address found in your NDR, then follow the on-screen instructions to request removal.

Configure SPF, DKIM, and DMARC

Access your domain's DNS hosting provider. Ensure that your SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC records are correctly published to verify your email source.

Monitor Outbound Traffic

Review your internal email server logs for any compromised user accounts, bulk mailing scripts, or malicious activity that might have triggered the spam filter initially.

Request Delisting from Microsoft and Authenticate Email

Delisting Timeline: Once the delisting request is approved by Microsoft Support, it may take up to 24 hours for the changes to fully propagate across Exchange Online Protection.

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Frequently Asked Questions

Why did Exchange Online Protection block my IP address?

EOP blocks IP addresses if it detects a high volume of suspected spam, malicious activity like malware distribution, or if the IP has a poor reputation on global blocklists.

How long does it take for Microsoft to delist an IP address?

After submitting a request through the Office 365 Anti-Spam IP Delist Portal, it typically takes a few hours to 24 hours for the block to be fully lifted across all Microsoft servers.

Can a missing SPF record cause error 5.7.511?

Yes, failing to authenticate your emails with standard protocols like SPF and DKIM makes your messages look suspicious to Exchange Online Protection, significantly increasing the risk of an IP block.

What should I do if my IP gets blocked again after delisting?

If the block returns, thoroughly investigate your network for compromised devices, secure your email accounts with strong passwords and 2FA, and ensure no bulk unsolicited emails are being sent from your domain.

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