Outlook Authentication Prompt When Opening HTML Email Containing HTTPS Images

Question

I am receiving HTML emails from a third party vendor MailChimp. Often, these emails cause Outlook (Microsoft Office Professional Plus 2016, (16.0.5095.1000) MSO (16.0.5095.1000) 32-bit to display an authentication prompt. I know I can use the registry setting AllowImageProxyAuth = 0 to hide the prompt, but I would like to at least understand the root cause for the login prompt. What I know: Only Outlook for Windows exhibits this behavior; not Outlook for the Mac, nor any browser such as Edge. The problem only affects HTTPS images from MailChimp. HTTP URLs do not generate the authentication prompt. I have created a test email using HTTPS images from another site and the authentication prompt does not appear. I send my test emails from Linux using /usr/lib/sendmail . Why does Outlook prompt the user to login when opening MailChimp generated emails?

This application does NOT permit me to attach the body of a test email. If you respond to this ticket with an email address, I will send you the troublesome email.

I have looked at the network traffic via

• Wireshark
• OpenSSL -s_client
  With no luck.

Answer 1

Hi @John Volmer ,
Welcome to our forum. Based on my research and tests, Outlook client would display a crendential prompt when you access the Internet by using a proxy server that is configured to use basic authentication and receive a HTML-formatted email message contains an HTTP reference to an image that requires proxy authentication, then we could manage teh prompt via register key "AllowImageProxyAuth", which may cause your issue.

I have created a test email using HTTPS images from another site and the authentication prompt does not appear. I send my test emails from Linux using /usr/lib/sendmail

And accroding to your description, the issue only occurred on these messages from Mailchip, so I'm afraid this issue is more related to these messages themselves , and there seems to be no option in Outlook client could change it. I also have tried to research a lot about your issue in the point of Mailchip, but haven't found any related information, in order to solve your issues better, it's suggested to contact with Mailchip support to get more information about it and check if you could change your message in the point of Mailchip.

Hope that would be helpful to you.

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Answer 2

You may close this. I have run out of time to investigate it.