Well thanks nudel, but of course .eml and .mht normally work like a charm in the Dopus preview pane. These are standard Internet Explorer formats that are handled very well by the ActiveX viewer plugin (however not at all by the MultiView plugin + QuickView DLLs).
Only thing is, nobody seems to know about it and therefore also the standard file format for .eml and .mht in Dopus is wrongly configured such that the preview doesn't work initially.
Usually it was just a matter of changing the file type "message/rfc822" for .eml and .mht in Dopus and the preview would work.
Unfortunately I don't remember anymore what file type made the preview work -- and a recent install and uninstall of IE7 seems have to mixed up these settings.
Hopefully somone who has a working .eml and .mht preview in Dopus can give me the right settings for it,
Since I've just lost the MHT preview in Dopus again, which happens to me about every couple years, and since I myself seem to be the only source for this problem on the Internets, here's a screenshot of how I solved it this time. Note that it only works when added to IE 64-bit, not when added to IE 32-bit.
If that's missing or disabled (or has the file extension removed), the 32-bit Internet Explorer line also has the MHT extension by default and still works when I test it today in Windows 10. The 64-bit version of IE should also work, but isn't the normal one to use.
Something seems to have broken parts of Internet Explorer and the underlying HTML components on your machine.