Many years ago (ca. 2009?) at a previous employer we had an enterprise email scanner which included a content filtering feature which could block "pornographic images". We ended up turning it off as it had too many false positives since it was an insurance company that dealt with injuries so a lot of legitimate injury photos would get classed as porn and blocked. I think it was basically looking at how much of an image was skin tones.

The more interesting thing about that feature was that it A) only blocked nude images of white people, and B) would aggressively block photos of golden retrievers much to the annoyance of someone in the claims team who bred them and occasionally received photos to her work email.