What percentage of all email is spam?

How to get close to this number

The answer has to be large but not overwhelming, and reasoning about the economics tells you why. Spam is nearly free to send, so senders will keep going as long as any fraction responds — which pushes the share upward. But filtering makes delivery unreliable enough that the share cannot approach totality. Those two forces settle it somewhere near half.

The trend is what surprises people, and it runs downward. As the explanation says, the share was well over seventy percent a decade earlier, and it fell because filtering improved and because legal action shut down the largest operations. Spam is one of the rare internet problems that has genuinely got better rather than worse.

The reason the figure feels wrong is stated in the explanation and is worth sitting with: you almost never see it. A statistic about how much of something exists is very hard to intuit when the entire purpose of the system around it is to hide it from you. Your inbox is evidence about filtering quality, not about spam volume.