Email seems to be an analogy for communication or perceiving information, or at least a predecessor of more fast-paced forms of that.

Could it be these new forms are so efficient and fast-paced, they overload our brain? Before the TV (which had limitations at start as well), there was telephone and radio. Both were limited. Telephone was expensive (monopoly yada yada), and radio informative (before the ~60s).

What Knuth did is enforce snail mail. This creates an artificial barrier, increasing signal to noise ratio.