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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