A user starts with a master password which is an authorized access key for a sender on a remote mail system to send a mail. The master password is automatically replaced by an authorized-to-reply key transmitted to the remote mail system. The mail systems are using the authorized-to-reply key and the permission-to-transmit key in a two-ways communication network to exchange mails. The master password is deleted later to prevent unauthorized use, without affecting previously established mail exchanges. An unknown sender requests an authorized-to-reply key through an interactive examination. A command flag enables the user to switch between anti-spam mode and non-antispam mode, and to rebuild the authorized-to-reply key in a security-demanding environment. The authorized-to-reply key is sub-divided to comprise a unique group name to facilitate sorting and graphic displays of the various groups. A utility program searches and matches all desired services and advertisements with the offers from other mail users.