Electronic mails (hereinafter, referred to as e-mails) have been communicated through the Internet, which is a standardized and global system of interconnected computer networks linking a great number of people worldwide, by users widely scattered from an initial or early stage of spread of the Internet. In a concrete practical manner of e-mail communication, each of users makes a membership registration contract with an e-mail service provider which carries on business with the operation of an e-mail server so as to become a member affiliated with the e-mail server and then makes access to the e-mail server on the contract by making use of a client program called MUA (Mail User Agent) installed in his or her own terminal apparatus (hereinafter, referred to as a member's terminal apparatus) to transmit or receive e-mails. (Hereinafter, “the member” means “the user” who has made the membership registration contract with the e-mail service provider.)
In the e-mail server which is used in a usual e-mail system wherein the member's terminal apparatus is constituted with a personal computer, a program called MTA (Mail Transfer Agent) for transmitting, receiving or transferring the e-mails, a program called MDA (Mail Delivery Agent) for delivering received e-mails to mailboxes (inner memory areas) each allotted to the member, a program called MRA (Mail Retrieval Agent) for transferring the received e-mails from the mailbox to the member's terminal apparatus, a program called MSA (Mail Submission Agent) for receiving e-mails from the member's terminal apparatus and so on are installed. The MTA is often divided into a MTA for transmission and a MTA for reception to be separately installed so that a situation wherein the MTA is used for illegitimate relay of the e-mails by a mailer of an unwanted or unsolicited e-mail (hereinafter, referred to as “a SPAM”) as described later can be prevented from being brought about.
The member is authorized an account with which he or she is able to make access to the e-mail server to which he or she is affiliated. In the e-mail server, account validation for recognizing the account of the member is conducted in the SMTP (Simple Mail Transfer Protocol) validation system for preventing the account of the member from being illegitimately used by the mailer of the SPAM when the member's terminal apparatus is connected with the e-mail server. With the SMTP validation system, the account validation is conducted with information representing the account of the member and a password given to the member on the strength of an extended function of the SMTP whenever the member's terminal apparatus is connected with the e-mail server.
Usually, the member obtains information representing a domain name or an IP (Internet Protocol) address for the MTA and MRA from the e-mail service provider with which he or she has made the membership registration contract and further is provided with information representing his or her own account, the password and an e-mail address by the e-mail service provider. The e-mail address is represented in the form of “mailbox name @ domain name”. The @ is so-called “at mark” (“at symbol”). The mailbox name is represented with a plurality of alphabetical letters peculiar to the account and placed to precede the @. The domain name is represented with a plurality of alphabetical letters peculiar to the e-mail service provider and placed to follow the @.
In the case where the member receives an e-mail from one of communication mates, for example, first, the member notifies the communication mate of his or her e-mail address, and then, the communication mate transmits the e-mail directed to the member's e-mail address through an e-mail server of an e-mail service provider with which the communication mate has made a membership registration contract to the e-mail server of the e-mail service provider with which the member has made the membership registration contract. In this case, the MRA in the e-mail server of the e-mail service provider with which the member has made the membership registration contract is operative to transfer the e-mail transmitted from the communication mate to the MUA installed in the member's terminal apparatus connected with the e-mail server of the e-mail service provider with which the member has made the membership registration contract, so that the e-mail transmitted from the communication mate can be read by the member. The connection of the member's terminal apparatus with the e-mail server of the e-mail service provider is allowed when the account validation is successfully conducted with the information representing the account of the member and the password given to the member.
In the meanwhile, in the case where the member transmits an e-mail to one of communication mates, for example, first, the member obtains the e-mail address of the communication mate, and then, the member transmits the e-mail directed to the e-mail address of the communication mate through the e-mail server of the e-mail service provider with which the member has made the membership registration contract to an e-mail server of an e-mail service provider with which the communication mate has made a membership registration contract. In this case, the MRA in the e-mail server of the e-mail service provider with which the communication mate has made the membership registration contract is operative to transfer the e-mail transmitted from the member to the MUA in an e-mail terminal apparatus of the communication mate connected with the e-mail server of the e-mail service provider with which the communication mate has made the membership registration contract, so that the e-mail transmitted from the member can be read by the communication mate.
Incidentally, in the case of a kind of e-mail called usually a web mail, a web browser installed in the member's terminal apparatus serves as the MUA and the MTA and MRA are installed in a web server. Further, in the case of an e-mail communicated through portable telephones (mobile phones), the MUA is installed in a member's portable telephone.
The substance of the e-mail is an electronic message constituted with a header portion containing various kind of information representing a source e-mail address, a destination e-mail address, an e-mail address for reply and so on and a body portion containing an electronic information message and an attached electronic file. When the e-mail is transferred from the MUA to the MTA or from the MTA to the MUA, an envelope containing a sender's e-mail address and an addressee's e-mail address is added to the e-mail.
The addressee's e-mail address contained in the envelope is set automatically by the MUA or the MTA. Although an e-mail address identical with the destination e-mail address is usually set as the addressee's e-mail address, it is not necessarily so.
When the member receives the e-mail from the e-mail server, he or she is can not obtain directly the addressee's e-mail address. Since the addressee's e-mail address is self-explanatory to the member, it does not bring about any problem that the member who receives the e-mail from the e-mail server can not obtain directly the addressee's e-mail address.
The sender's e-mail address contained in the envelope is also set automatically by the MUA or the MTA. Although an e-mail address identical with the source e-mail address is usually set as the sender's e-mail address, it is not necessarily so. The source e-mail address is voluntarily described by the sender for convenience' sake of the addressee and not indispensable to the delivery of the e-mail.
When the member receives the e-mail from the e-mail server, he or she is can not obtain directly the sender's e-mail address. The sender is usually confirmed with the source e-mail address.
In general, the e-mail service provider provides each of the members with a plurality of e-mail addresses within a predetermined number. The e-mail address given to the member is notified to others mainly in the manner of a direct notice, an indirect notice, a notice through database registration, a notice through mailing list registration or a public notice.
In the case of the direct notice, for example, an e-mail address of a member A is directly notified by the member A to an acquaintance B of the member A. As a result, the member A is able to receive an e-mail from the acquaintance B.
In the case of the indirect notice, for example, the e-mail transmitted from the member A having his or her own e-mail address to the acquaintance B of the member A is further transferred from the acquaintance B to an acquaintance C of the acquaintance B, so that the e-mail address of the member A is notified to the acquaintance C of the acquaintance B. As a result, the member A is able to receive an e-mail also from the acquaintance C of the acquaintance B.
In the case of the notice through database registration, for example, the member A having his or her own e-mail address notifies a dealer D of the e-mail address to be registered on a database managed by the dealer D in order to receive a communication from the dealer D when he or she wants to purchase a product supplied by the dealer D, and then, the e-mail address of the member A is notified through the database managed by the dealer D to others belonging to the database.
In the case of the notice through mailing list registration, for example, the member A having his or her own e-mail address registers the e-mail address in a mailing list wherein e-mail addresses of specific members are registered in order to make information exchanges among the specific members, and then, the e-mail address of the member A is notified to all the specific members each having the e-mail address registered in the mailing list through the following transmission and reception of the electronic mail messages. In general, when one of the specific members each having the e-mail address registered in the mailing list transmits the e-mail directed to the e-mail address of the mailing list, the e-mail thus transmitted is delivered to all the specific members each having the e-mail address registered in the mailing list. Such an e-mail is usually provided with the e-mail address of the mailing list as the e-mail address for reply. Further, the destination e-mail address is not changed usually and therefore each of the specific members each having the e-mail address registered in the mailing list receives the e-mail provided with the e-mail address of the mailing list as the destination e-mail address. In addition, the source e-mail address is also not changed usually and therefore the e-mail address of the sender of the e-mail directed to the e-mail address of the mailing list is notified to all the specific members each having the e-mail address registered in the mailing list.
In the case of the public notice, for example, the member A having his or her own e-mail address establishes his or her own web site to manage the same and opens his or her e-mail address to the public in the web site, so that the e-mail address of the member A is notified to many and unspecific persons each making access to the web site managed by the member A. As a result, the member A can receive e-mail messages from many and unspecific persons.
After the member's e-mail address has been notified to others as mentioned above, an undesirable person is able to know also the member's e-mail address through a route which is not under the control by the member. Accordingly, it is likely that a large number of SPAMs including advertising e-mails which are automatically transmitted to unspecified e-mail addresses without confirming addressee's intention of receiving the same and wicked e-mails veiling fraudulent intentions are delivered to the member's e-mail address. Further, it is also likely that the SPAMs transmitted to addressee's e-mail addresses listed irresponsibly are delivered to the member's e-mail address which is accidentally identical with one of the addressee's e-mail addresses listed irresponsibly. These SPAMs delivered to the member's e-mail address inflict serious damages on the member. For example, the member is subjected to a lowering of working efficiency, oversights of necessary e-mails, fraudulent practices, an increase in cost for communications through a portable telephone brought about by receiving useless packets and so on by the SPAMs delivered to the member's e-mail address.
There have been previously proposed various kinds of measures against such SPAMs as mentioned above. In one of the previously proposed measures against the SPAMs, e-mails each containing predetermined key words are judged to be SPAMs and refused by the MTA. In the case where such a measure against the SPAMs is adopted, if a criterion for judgment of the SPAM is severely set, a problem that a necessary e-mail which is not the SPAM is wrongly judged to be the SPAM so as to be refused by the MTA is brought about. Further, in this case, there is room for allowing a SPAM mailer to transmit a SPAM which pretends a normal e-mail to be received by many and unspecified persons.
In another of the previously proposed measures against the SPAMs, e-mails each transmitted from a specific source e-mail address are judged to be the SPAMs and refused by the MTA. In the case where such a measure against the SPAMs is adopted, a black list on which e-mail addresses of specific persons who are deemed SPAM mailers are registered is provided and e-mails each having a source e-mail address identical with the e-mail address registered on the black list are judged to be SPAM and refused by the MTA. In this case, first the reception of a relatively large number of SPAMs is confirmed at least once and then the e-mail addresses identical with the source e-mail addresses of the SPAMs are registered on the black list. Therefore, there is a problem that the measure against the SPAMs is undesirably forestalled.
In addition, there has been also proposed a supplemental measure against the SPAMs wherein the member is provided with a plurality of e-mail addresses from the e-mail service provider and uses the e-mail addresses each in its proper way. When a predetermined number of SPAMs each directed to one or more of the e-mail addresses given to the member have been received, the one or more e-mail addresses are discontinued and one or more new e-mail addresses are provided in place of the discontinued e-mail addresses. Each of the e-mail addresses given to the member is under the member's own management.
For the purpose of improvement in efficiency of the measure against the SPAMs wherein each e-mail address is used in its proper way, it has been continued to develop measures for managing unitarily corresponding relations each between each of the e-mail addresses given to the member and a communication mate. In one of such measures already developed, for example, variable e-mail addresses each having a mailbox name containing a variable portion corresponding to a single communication mate or a communication mate in the form of a group of persons and hard to analogize are provided and the management of the corresponding relation between each of the variable e-mail addresses and each of the communication mates is carried out in the e-mail server or the member' terminal apparatus so that the corresponding relation between each of the variable e-mail addresses and each of the communication mates is linked to the information of the communication mate recorded in a communication history, an address note and so on (as disclosed in, for example, patent document 1).
Such a measure as disclosed in patent document 1 belongs to a white list system wherein a white list on which specific e-mail addresses listed up are registered is provided and only an e-mail from a source e-mail address identical with one of the e-mail address registered on the white list is permitted to be received. The measure against the SPAMs of the white list system is highly efficient in function or performance for preventing the SPAMs from being received. However, in practice, there is a problem that the member has to carry out continuously a complicated management of corresponding relations each between the member's e-mail address and each of the e-mail addresses of the communication mates. Further, there is another problem in the case where each e-mail address given to the member is used in its proper way for each communication mate that the member's e-mail address must be notified to each of the communication mates in the manner secret to a third party, that is, the member's e-mail address must be surely prevented from leaking out.
To solve the problems mentioned above, there have been further proposed several improved measures against the SPAMs. In one embodiment of the improved measures previously proposed, a source e-mail address contained in a header portion of an e-mail is converted into an e-mail address registered previously for each communication mate (as disclosed in, for example, patent document 1). In another embodiment of the improved measures previously proposed, a center e-mail server for supplying the member with e-mail addresses and notifying communication mates of the member of the e-mail addresses given to the member is established (as disclosed in, for example, patent document 2). In a further embodiment of the improved measures previously proposed, in addition to provision of variable e-mail addresses used in the white list system, fixed e-mail addresses each having a mailbox name containing a fixed portion to which a pass code for receiving an e-mail from an unspecified person is set are provided and an e-mail directed to the fixed e-mail address is permitted to be received only when a pass code transmitted together with the e-mail directed to the fixed e-mail address is identical with the pass code set to the mailbox name of the fixed e-mail address (as disclosed in, for example, patent document 3).    Patent document 1: Japanese Patent Publication No. 2003-196216    Patent document 2: Japanese Patent Publication No. 2003-323379    Patent document 3: Japanese Patent Publication No. 2004-102352