1. A method of registering an electronic-mail (e-mail) sender for the purpose of sending e-mail messages to an e-mail recipient, the method comprising: receiving an e-mail sent by the e-mail sender to the e-mail recipient; executing instructions stored in memory, wherein execution of the instructions by a processor of an independent third-party auditor generates a challenge message in response to an e-mail message received by the e-mail recipient, wherein the challenge message does not invite a response to the challenge message from the e-mail sender; sending the challenge message to the e-mail sender; receiving an error message associated with the e-mail sender within a predetermined period of time after the sending of the challenge message, the error message suggesting that the e-mail sender is not a valid e-mail address where mail can be sent, wherein the error message is evaluated by the independent third-party auditor; and sending a report generated by the independent third-party auditor to the e-mail recipient, the report indicating receipt of the error message, wherein the e-mail recipient classifies e-mail messages from the e-mail sender based on the report.