Hi Family
Again having issues with the bool tool in 4.2.1, it simply will not bool the mug to a cube for mould making. I include a link to the file Unique Download Link | WeTransfer
Looks like the possible problem is on the mug object itself. It has really messy geometry/topology, perhaps result from a 3D scan? There are lots of problems like flipped/inverted normals particularly on the extruded parts near the bottom, inner faces, overlappping geometry, not to mention it’s really dense.
Things like these would be problematic for boolean operation, so having a clean topology could do a great help, preferably by making/adding new geometries that closely follow the shapes as cleaning up manually might become difficult and tedious.
Thank you so much for taking the trouble to look at the piece and your very insightful explanation. I had a feeling it could be that section causing the problem and will go look at it with this information and advice you gave.