configure the IIS server to deny requests made without the Host header set.

Hi @wei liu,

Welcome to the Microsoft Q&A platform!

Based on your description, the configuration in IIS that denies requests without a Host header is interfering with your POP3 service. This happens because some services, such as POP3, may not include a Host header in their requests, causing them to be blocked by IIS.

There are several steps you can take to resolve this issue:

1. You can configure IIS to allow requests from the POP3 service by creating exceptions. This can be done by modifying the request filtering settings to exclude certain IP addresses or request types.

2. Modify the request filtering settings:

• Open IIS Manager.
• Select the server or site for which you want to configure settings.
• In Features View, double-click Request Filtering. 
• In the Actions pane, click Edit Feature Settings.
• Adjust the settings to allow requests without a Host header for specific services or IP addresses.

3. If you have the URL Rewrite module installed, you can create a rule to bypass the Host header check for POP3 requests.

• Open IIS Manager.
• Select the server or site.
• In Features View, double-click URL Rewrite.
• Add a new rule to allow requests without a host header for the POP3 service.

4. Make sure your POP3 service is correctly configured to work with IIS. Sometimes adjusting the service settings can solve the problem.

Please feel free to contact me for any updates. And if this helps, don't forget to mark it as an answer.

Best,

Jake Zhang