The idea is you'd have an email client but all data is stored encrypted on chain and you'd have to pay tx costs to send things. By having this sort of stamp tax spammer wouldn't be able to scale mass messaging as it would bankrupt them.
in meatspace though, my real mailbox is mostly filled with spam that the sender paid to have delivered.
granted, it would at least act as a limiting factor.
If you have an auditable tx history, then mixers/exchanges/wallets etc can be marked as sources of the money used by spammers