Provides e-mail address forming methods to know with certainty whether or not an e-mail address was leaked. A method includes: sending a receiver's identifier and a sender's identifier to a receiver's mail server; computing a value which is encrypted by the mail server with a secret key, the secret key being only possessed by the mail server, from the receiver's identifier, the sender's identifier, and a nonce issued by the mail server, and sending the value to a receiver; and forming an e-mail address (LD address) to be used by a sender who sends a mail to a receiver, by attaching a receiver's domain name to the encrypted value. Furthermore, the present invention has an e-mail address sending method, and en e-mail sending system which uses the e-mail address forming method to know with certainty whether or not the user of an e-mail address leaked the e-mail address.