I have been reading the Hashcash paper which is referenced in the Bitcoin whitepaper. I would like to know what is the current state of the Hashcash protocol and what are the services that are actively using it?
There is no serious usage, and has never been, of Hashcash.
Most obviously, the disparity between the resources of a spammer (who can rent a botnet of stolen compute resources, or use high powered servers and GPUs) is no match for a user sending an email from their phone with limited power available . This simply means that the difficulty of generating a "stamp" has to be so high that only spammers are able to send messages, which defeats the purpose.