My web server examines the the referer to deny deep linking to images on my sites, so I get log messages about that. When I browsed the logs today a few of those log lines caught my eye. They looked like this:
client denied by server configuration: /directory/image, referer: junomsg://028985E8/
(I replaced the filesystem path to the image with /directory/image.)
What kind of protocol is junomsg? I did some research and it seems like this comes from the “Juno offline email client”. Someone probably received a link to my site by mail and the offline client seems to run in Internet Explorer. Case closed.