>I could not find this option in Outlook. I checked Help, it says: Go to Tools\Options\Other\Advanced Options\Custom Forms\Web Services and adjust here, but I don't know our Web Services Location and I doubt, this is the way I should set it - seems too complicated...

Well, there is definitely something in regards to this recent setup. We ship regular HTML emails at large in various forms. I would still be curious to receive a snapshot by email however of what you are seeing.