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
- OpenSSL -s_client
With no luck.