Systems and methods are disclosed for preventing spam, including email spam and telephone spam, through the use of dynamic passcodes or dynamic signatures included with transmitted messages. Both the dynamic passcodes and dynamic signatures may be changed periodically or continually, according to an example embodiment of the invention. Qualified senders may include a dynamic signature that is automatically generated by the sender's node or exchange entity. The recipient's node or exchange entity can then use a graylist associated with the recipient account in verifying the qualified sender and/or dynamic signature. On the other hand, non-qualified senders may need to manually obtain a dynamic passcode from a dynamic passcode provider and include the dynamic passcode with the transmitted message. The dynamic passcode provider may be a public website using human interactive proofs. According to an embodiment, a non-qualified sender may be optionally pre-charged with a fee to obtain a dynamic passcode. The recipient of the message may then refund the charge if the received message is not spam.