If I open an EML file (mail) on the viewer and try to delete it from there, I receive an error message about the file being open by "Directory Opus ActiveX Proxy".
That is not what I expect to see. I would expect it to behave like when I delete a picture.
I think it is a bug. Please try to reproduce it and fix it.
I've checked here and Opus 12.3.4 correctly tells the viewer to close before trying to delete the file (as long as I delete it from Opus; it can't know something else is about to delete the file). EML files visible in the viewer delete OK for me, using the same Windows Mail MIME E-Mail preview handler (via the ActiveX plugin).
But some of the EML-related components in Windows have had longstanding bugs where they leave files locked, so you may be running into something similar, or something may be causing the viewer not to close in a timely manner. Telling the ActiveX plugin to pass a temporary copy of the file to the Windows Mail MIME E-Mail preview handler works around the problem by leaving the temp file locked instead of the real one.
In fact, you could say there is definitely a bug here, in the Windows Mail MIME E-Mail preview handler, because the specification for preview handlers states that they should not leave files locked while viewing them, only for as long as it takes to load them. But a lot of preview handlers break that rule, unfortunately. (Almost all of the PDF ones, for example. But we force those to open temporary copies.)