I tried to export a drawing as a DXF.
In the export dialog explicitely selected "base unit" mm. The unit of the document is also mm.

If I import the Inkscape DXF export into a CAD-Tool I get dimensions which are a factor 3.543 too small.
After guessing and messing around, I think I know where this number comes from:
3.543=90/25.4, obviously coming from 90 dpi of the screen and 25.4 mm/inch.

How to get the correct dimensions?
Well, the answer seems to be: export the drawing with base unit "px".

Now, I found that I am not the first one pointing to this issue.
Should this be reported as a bug or will it stay a feature?

Might depend on the CAD software...

My CAD (Geomagic Design) offers input base units. I select mm and all is good.