Every player can configure an amount of ingame money that is levied from a sender's account to deliver a message to them. It's a currency sink, so it's themed as a "tax" levied by the NPCs and its value is destroyed rather than paid to the recipient of the message.
I thought it used to default to 5 ISK (a pittance, something you can make back by shooting a single NPC pirate ship). I see some references to the default being ~2000 ISK at the time that it was changed to 0, where it is now.
Worked pretty well, imo. Players that need to be publicly contactable (people who organize public events, for instance) can turn it off easily. People who are "space famous" can crank it up to reduce targeted spam. Even at the default setting, it's effective at keeping ingame scammers from blasting the whole player list with messages (at least, the poor ones :). Doesn't apply to people you've already exchanged messages with. I think there's also some allowlisting you can do, etc.
0. https://wiki.eveuniversity.org/EVE_Mail#CONCORD_Spam_Prevent...
The idea in general seems to have been around since 1992: https://en.wikipedia.org/wiki/Hashcash