I'm curious about this as well so will be learning more about dapps (decentralized apps). Starting here

https://blog.chronobank.io/what-is-a-decentralised-applicati...

https://ethereum.org/en/developers/docs/dapps

For messaging dapps, check out the new Blockscan chat by the creators of Etherscan : https://decrypt.co/91226/etherscan-creators-launch-ethereum-... (note: NOT very decentralized yet)

This is past my level of expertise, but you could map out what those clock cycles of a mail server are going towards. For example, serving files, like we talked about, can happen on IPFS. No compute needed there beyond what IPFS already does.