While in the original design, SMTP email is like writing on a postcard in light pencil and throwing it into the wind, in present reality the infrastructure has

While in the original design, SMTP email is like writing on a postcard in light pencil and throwing it into the wind, in present reality the infrastructure has solidified quite nicely. What's now novel about it is that it's selectively reliable -- people can just not send or reply as expected, and generally get away with it.