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JohnVolmer-9732 avatar image
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JohnVolmer-9732 asked JohnVolmer-9732 answered

Outlook Authentication Prompt When Opening HTML Email Containing HTTPS Images

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.51483-mailchimpcertificatepromptv02.png I know I can use the registry setting AllowImageProxyAuth = 0 to hide the prompt, 51514-outlookregistry20201124a.png 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.



office-outlook-itpro
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Hi @JohnVolmer-9732,
It seems to you haven't upload the screenshots successfully in your post, could you please edit your post and re-add them for us to further confirm your issues?(Notice: Please hide your personal information in the screenshot for security.)

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I'll upload the screen shots again:

55554-52709-mailchimpcertificatepromptv.png
52744-outlookregistry20201124a.png



Unfortunately this application still refuses top let me append the actual HTML email.


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Another observation: the specific authentication prompt (shown below) depends on the how the user logs in. I normally log in with a smart card, so I was asked to login with my smart card. If I had logged in with userid/password, I would have a received a prompt for userid/password.

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JadeLiang-MSFT avatar image
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JadeLiang-MSFT answered JadeLiang-MSFT commented

Hi @JohnVolmer-9732,
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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Thank you for your quick response. Alas, a colleague of mine has contacted Mailchimp and their response is "you are the only customer that has reported this issue. The problem must be at your end." So I am looking for more concrete evidence as to where the problem resides.

This problem also occurs when their is NO proxy in the path.

I suspect that AllowImageProxyAuth only hides the onerous symptom of the problem; it does not actually address the problem.

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Thanks for your update, and I'm still researching for your issues and searching a lot about it.

According to your description, in my opnion, I think it may be related to theses messages that containing https images may need to be verified for security in Outlook, but I haven't found the specific aiticle that could confirm it. As for Mailchimp, due to we couldn't test with it, we may not be able to provide with more information about it.

So, if you indeed wouldn't like to see this prompt, changing the register key as you mentioned may be a necessary option to achieve it. But I still think this prompt is used for protecting your windows and Outlook, and it's more recommended to enable it. Besides, for the specific reason or working principle of it, I would continue to research for it, and if I have any update in the future, I would post back here.

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Thank you for pursuing this.

I do have a stripped down HTML email that usually replicates the issue. If you want it, let me know how to get it to you.

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Hi @JohnVolmer-9732,
Thanks for your update, please kindly understand that here we could only provide support in forum and may not be able to share the email file.
Have you tried to test on other client? And could this issue be reproduced on others? If it also would reproduce on other client, we may preliminarily exclude the reason of Outlook client.

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The reports I have received from my colleagues is that no other mail client exhibits this authentication prompt, including Outlook for the Mac. Nor does any browser. For example, Mail (Version 16005.13426.20316.0) on Windows 10 does not.

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I was able to capture an Outlook log file that precedes the authentication prompt. But, I cannot find a description of the Outlook events. Do you know where Outlook log events are documented? Thanks!

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Thanks for your update, please kindly understand that here we mainly focus on general issues on Outlook desktop client and couldn't provide the support of Outlook log.
For your requirement, I have tried to researched a lot about Outlook log event and found a related article, which mentioned we could check Outlook event via Event Viewer that could provide the general information about the event. (Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
Hope that would be helpful to you.

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Just cheking if your issue has any update, if you don't have any other questions in the point of Outlook client, in order to close this thread normally, could you mind marking the helpful post or your own post to anwer?

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JohnVolmer-9732 avatar image
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JohnVolmer-9732 answered

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

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