The server would allow the user to request a ticket in exchange for a proof of work: expending CPU cycles solving hard algorithms with processing power, Turing tests, or even just by paying money. The server could also cancel a ticket. For example, after receiving an email with a ticket, the receiver could request the ticket to be cancelled so it cannot be reused. The person who cancels a ticket also has the option to refund the ticket to the sender. This causes the original sender to regain a new ticket. For example, a user might refund a ticket that came with an email if it was not spam.