Patent ID: 7590695

Claim:
In a computer system having a processor and memory that monitors electronic messages that are sent from senders to recipients, a method of determining whether an electronic message has been sent from an authorized sender, comprising: receiving an electronic message that includes information identifying a sender of the electronic message; transmitting to the sender a first challenge message; specifying a predetermined time after an event from which a first response to the first challenge message should be received for the purpose of requiring the sender to spend a certain amount of time and requiring a certain amount of computing resources of a system of the sender to prepare the first response to the first challenge message such that responding would be impractical for a bulk messaging system; receiving the first response to the first challenge message; when the first response to the first challenge message is correct and when the first response to the first challenge message is received after the predetermined time from the event, transmitting to the sender a second challenge message after a predetermined period of time has passed since transmitting the first challenge message, and specifying that receipt of the first response to the first challenge message and a second response to the second challenge message must be separated from each other by at least a requested interval for the purpose of requiring the sender to spend a certain amount of time and requiring a certain amount of computing resources of the system of the sender to prepare the second response to the second challenge message such that responding would be impractical for a bulk messaging system; receiving the second response to the second challenge message; and when the second response to the second challenge message is correct and when receipt of the first response and the second response are separated from each other by at least the requested interval, designating the sender as being authorized to send electronic messages to the recipient, the sender being designated as being authorized only when the sender provides appropriate responses to the first and second challenge messages.