Patent ID: 7711786

Claim:
A spam-prevention method, comprising: receiving, at an exchange entity for a recipient, an electronic message that includes a source address associated with a sender and a dynamic tag, wherein the dynamic tag includes a first dynamic code and a counter; locating a hashing function and a secret key; determining a second dynamic code using at least the hashing function, the secret key, and the counter of the dynamic tag; verifying the dynamic tag by matching the first dynamic code of the dynamic tag to the second dynamic code; and accepting the electronic message to an account of the recipient upon verifying at least the dynamic tag, wherein the sender is a non-qualified sender, wherein the dynamic tag is a dynamic passcode, wherein locating the hashing function and the secret key includes locating a system hashing function and a system key, and wherein the dynamic passcode is obtained by the non-qualified sender manually querying a public dynamic passcode provider, wherein the dynamic passcode provider is a Web server, and querying the dynamic passcode provider includes the non-qualified sender entering at least a static email address of the recipient and completing a human interactive proof, wherein the dynamic passcode is generated by the dynamic passcode provider applying the system hashing function to a concatenated string comprising at least a portion of the static email address of the recipient, the system key, and an incremental counter associated with the recipient, wherein the prior steps are performed by one or more computers.