How would you feel about patenting language? I.e. If you speak with certain words or certain patterns then you have to pay a royalty (only for 2 to 5 years).
There is more than twice spam as there is legitimate email (source: ran a corporate email server). However, so far most spam is automatically detected.
In the near future, It's likely that this is no longer going to be the case and in the moment that most of your email is spam you are going to stop trusting everything you receive.
Even right now inside corps people already (should?) distrust and double checks everything that comes from another domain.