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How to Investigate Unexpected Microsoft Postmaster Complaint Emails

Maira Mehtab Sep 22, 2026 869 views

Question details

The user needs to investigate unexpected complaint messages sent from a masked Hotmail address regarding an unknown server network not listed in their Microsoft SNDS access list.

Product: Microsoft Postmaster / Email Services
Device & OS: not provided
Scenario: Receiving unusual server complaint emails directed at a company email account from an unrecognized network.
Observed behavior: Unexpected postmaster complaint emails are being delivered to the company account, referencing a server network that does not appear to belong to the organization's SNDS access list.

Before you start

Before proceeding, ensure you have access to the raw message source of the suspicious email and can access a reliable web-based email header analyzer.

Solution 1Recommended

Analyze Message Headers and Verify Domain Records

Use a header analyzer to determine the true origin of the complaint email and check your domain's authentication settings to rule out spoofing.

Unexpected complaint messages may indicate issues with sender reputation, email spoofing, or spam filtering. By inspecting the full message headers, you can trace the exact originating IP address and verify if the referenced server actually belongs to your domain.

Obtain the raw email headers

Open the unexpected complaint email in your email client and locate the option to view the raw message source or full internet headers (often found under 'Message Details' or 'Properties').

Analyze the headers

Copy the full internet headers and paste them into a header analyzer tool, such as the Microsoft Message Header Analyzer (https://mha.azurewebsites.net/), then click analyze.

Identify the originating IP

Review the analyzer output to identify the actual sender, originating IP address, and the complete mail path.

Verify server ownership

Determine whether the reported IP address or server belongs to your organization, your email service provider, or your domain. Compare it against your known Microsoft SNDS access list.

Check domain authentication records

Log in to your DNS provider's dashboard and verify your domain's SPF, DKIM, and DMARC records to ensure they correctly authorize only your actual sending servers.

Handle Unknown Servers with Caution: If the originating server is not associated with your organization, treat the messages cautiously. Do not click any internal links, and report suspected abuse or phishing to your relevant email provider.

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Frequently Asked Questions

What is the Microsoft SNDS access list?

The Microsoft Smart Network Data Services (SNDS) provides network owners with data about traffic originating from their registered IP space. It helps senders monitor their IP reputation, track email volume, and investigate spam complaints.

Why am I receiving complaints for an IP address I don't own?

This can happen if your email address is being spoofed by malicious actors, or if a third-party service provider is sending emails on your behalf using a server that you do not directly manage. Analyzing the email headers helps identify the true source.

How do SPF, DKIM, and DMARC prevent spoofed emails?

SPF specifies which IP addresses are authorized to send email on behalf of your domain. DKIM adds a digital signature to emails to prove they weren't altered in transit. DMARC uses both to instruct receiving mail servers on how to handle messages that fail authentication, protecting your domain's reputation.

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