TECHNICAL FIELD The present invention relates to a dual mail service system, and more particularly to a system and method for providing a dual mail service to users, wherein, when a user transmits a domain name of a mail server to a central mail server in order to send/receive mail to/from the mail server, the central mail server provides network access information of the mail server or the central mail server according to whether or not the mail server can provide a mail service.

BACKGROUND ART Along with the development of network technology, the application of email for communicating messages between individuals has been gradually increased, replacing conventional postal mail. Recently, email has come into popular use not only as personal mail but also for a variety of notifications and product advertisements. In today's society, email serves as the primary communication means due to ease and swiftness, i.e., due to advantages in that they can be composed with ease and convenience and can also arrive at recipients immediately after they are sent. Fig. 1 illustrates the configuration of a conventional mail system 100. The conventional mail system comprises a sender PC 101, a recipient PC 102, a sender mail server 103, a recipient mail server 104, and a network 105. Senders and recipients have opened mail accounts corresponding to their addresses in the mail servers 103 and 104. Each of the mail servers 103 and 104 is divided into SMTP and POP3 servers which use different protocols to send and receive mail. Mail is sent and received in the following manner. The sender PC 101 activates a mail program, receives a message and a recipient email address entered by the user, and sends a corresponding email. The outgoing mail is sent to the SMTP server in the sender mail server 103, and then arrives at the recipient mail server 104. The recipient PC 102 activates a mail program and receives mail arriving at its mail account from the POP3 server of the recipient mail server 104. Most users have joined large portal sites and have opened mail accounts in mail servers provided by the portal sites. Typically, users receive mail using their mail receiving programs through POP3 servers provided by mail servers, or log into large portal sites to check their mail through web pages of the portal sites. Nowadays, as client users increasingly implement web sites on their PCs, i.e., using their PCs as web servers, some users desire to implement not only web servers but also mail servers in their client PCs. However, some problems occur in such a conventional system in which a mail server PC, which is a general PC with a mail server implemented therein, provides a mail service to a number of user client PCs. For example, if the mail server PC is not available for whatever reason, users having mail accounts, who made requests to send/receive mail, must wait until the mail service of the mail server PC is restored. Particularly, although the user of the mail server PC uses it as a general PC, they must turn it on 24 hours a day to operate it as a mail server even when they do not use the PC. DISCLOSURE OF INVENTION Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a system for providing a dual mail service using a mail server PC and a central mail server, wherein the mail server PC is provided by implementing a mail server in a general PC, and, when a client PC transmits a domain name of the mail server PC in order to access the mail server PC, the central mail server transmits network access information of the mail server PC or the central mail server according to whether or not the mail server PC can provide a mail service, so that the central mail server substitutes for the mail server PC and provides a mail service to client PCs when the mail server PC cannot provide a mail service. In order to accomplish the above object, the present invention provides a dual mail service system, which includes a receive-only SMTP (Single Mail Transfer Protocol) server for receiving incoming mail from an external SMTP server corresponding to a sender of the incoming mail and transmitting the incoming mail to a corresponding central mail server; a send-only SMTP server for receiving outgoing mail from a central mail server and transmitting the outgoing mail to an external SMTP server corresponding to a recipient of the outgoing mail; at least one central mail server for sending mail to the send-only SMTP server and receiving mail from the receive-only SMTP server, assigning an individual user home directory and buffer to each of a plurality of mail server PCs (Personal Computers) to send/receive mail to/from the mail server PC, checking, in real time, whether or not the mail server PC can provide a mail service, assigning network access information of the mail server PC to a mail server address if the mail server PC can provide a mail service, assigning network access information of the user home directory to the mail server address if the mail server PC cannot provide a mail service, receiving a domain name of the mail server PC from a user client PC, and transmitting the network access information; at least one mail server PC including a general PC with a mail server implemented therein to send/receive mail over a network, the at least one mail server PC sending/receiving mail to/from the central mail server and providing a mail service for mail accounts requested by a plurality of user client PCs when the at least one mail server PC can provide a mail service; and at least one user client PC for transmitting the domain name of the mail server PC to the central mail server, receiving the network access information, accessing the mail server PC using the network access information to send/receive mail when the mail server PC can provide a mail service, and accessing the user home directory in the central mail server using the network access information to receive a mail service when the mail server PC cannot provide a mail service. According to one aspect of the present invention, the central mail server includes an incoming mail receiver for receiving incoming mail from the receive-only SMTP server; an incoming mail manager for recording reception information of the incoming mail in a user home directory of a mail server PC corresponding to the incoming mail, and storing a file of the incoming mail in a buffer; an incoming mail information DB in which the incoming mail manager stores the reception information including a recipient mail account, a title, a sender mail account, and a sending date of the incoming mail; an incoming mail buffer in which the incoming mail manager stores incoming mail files in the order in which the incoming mail files are received; an outgoing mail receiver for receiving outgoing mail sent by the mail server PC; a destination address searcher for determining whether a recipient mail account of the outgoing mail is a mail account of the mail server PC, which has sent the outgoing mail, a mail account of a different mail server PC, or an external mail account; an outgoing mail transfer unit for leaving the outgoing mail in the central mail server, transferring the outgoing mail to a different central mail server, or transferring the outgoing mail to the send-only SMTP server according to the determination by the destination address searcher; a network access information manager for checking, for all mail server PCs registered in the central mail server, whether each mail server can provide a mail service in order to set network access information, receiving a mail server address from the user client PC, and transmitting the network access information corresponding to the mail server address; and a network access information DB in which the network access information manager stores network access information of the mail server PC by associating the network access information of the mail server PC with the mail server address of the mail server PC when the mail server PC can provide a mail service and stores access information of the user home directory of the central mail server by associating the access information of the user home directory with the mail server address of the mail server PC when the mail server PC cannot provide a mail service. In addition, the mail server PC may includes an incoming mail receiver for receiving, in real time or at specified time intervals, recent incoming mail from the central mail server; an incoming mail transmitter for transmitting the incoming mail, received by the incoming mail receiver, over a network upon receiving a request from the user client PC; an outgoing mail receiver for receiving outgoing mail from a plurality of the user clients; and an outgoing mail transfer unit for transferring outgoing mail, received by the outgoing mail receiver, to the central mail server, wherein, when the mail server PC has recovered from a state in which the mail server PC cannot provide a mail service, the incoming mail receiver collectively receives recent incoming mail, which has not been received, from the central mail server. The dual mail service system using the mail server PC and the central mail server, which is configured as described above, ensures that ordinary people can use their mail server PC, which is a general PC with a mail server implemented therein, to provide a mail service to users and can also use it as a general-purpose client PC. In addition, when a mail account user transmits a domain name of the mail server PC to the central mail server, the central mail server transmits physical network access information of the mail server PC or the central mail server to the mail account user according to whether or not the mail server PC can provide a mail service. Accordingly, the mail account user receives a mail service by transparently accessing the mail server with the received network access information, which is a mail server PC address or a central mail server address, so that the user can stably send/receive mail through such a dual mail service of the mail server PC or the central mail server.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken accompanying drawings. In the drawings: Fig. 1 is a block diagram of a conventional mail system; Fig. 2 is a block diagram of a dual mail service system according to an embodiment of the present invention; Fig. 3 is a block diagram of a central mail server, which is implemented as an integrated server, according to an embodiment of the present invention; Fig. 4 is a conceptual diagram of an automatic access function of a user client PC according to an embodiment of the present invention; Fig. 5 is a detailed block diagram of a dual mail service system according to an embodiment of the present invention; Figs. 6a and 6b are diagrams illustrating mail transmitting/receiving environments in a dual mail service system according to an embodiment of the present invention; Fig. 7 is a flow chart of a dual mail service method according to an embodiment of the present invention; Fig. 8 is a flow chart of a procedure for registering a mail server PC in a central mail server according to an embodiment of the present invention; • Fig. 9 is a flow chart of a procedure for registering a user mail account in a mail server PC according to an embodiment of the present invention; Fig. 10 is a detailed flow chart of the step of checking, in a central mail server, whether or not a mail server PC is available according to an embodiment of the present invention; Fig. 11 is a flow chart of a procedure in which a mail server PC receives incoming mail from a receive-only SMTP server according to an embodiment of the present invention; Fig. 12 is a flow chart of a procedure in which a user client PC receives incoming mail according to an embodiment of the present invention; Fig. 13 is a flow chart of a procedure in which a user client PC sends outgoing mail according to an embodiment of the present invention; and Fig. 14 is a flow chart of a procedure in which a central mail server sends outgoing mail according to an embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Fig. 2 illustrates the configuration of a dual mail service system 1 according to an embodiment of the present invention. The dual mail service system 1 according to the present invention comprises a receive-only SMTP server 3, a send-only SMTP server 4, central mail servers 5, mail server PCs 6, and user client PCs 7. The receive-only SMTP server 3 manages a plurality of central mail servers 5, each of the central mail servers 5 manages a plurality of mail server PCs 6, and each of the mail server PCs 6 provides a mail service to a plurality of user client PCs 7. The receive-only SMTP server 3 receives incoming mail from an external SMTP server 2 corresponding to a sender, searches for a central mail server 5 to which a recipient mail account of the incoming mail belongs, and transmits the incoming mail to the found central mail server 5. The central mail server 5 receives the incoming mail from the receive-only SMTP server 3, searches for a corresponding mail server PC 6 using a mail address of the incoming mail, and stores the received mail in a user home directory of the found mail server PC 6. The central mail server 5 checks, in real time, whether or not the mail server PC 6 can provide a mail service, and manages actual network access information corresponding to a domain name of the mail server PC 6. In this manner, the central mail server 5 functions as a domain name server. The mail server PC 6, which is a general PC with a mail server implemented therein, accesses, in real time or periodically, the central mail server 5 and searches for and downloads recent incoming mail, which has not yet been downloaded. Depending on whether or not the mail server PC 6 can provide a mail service, the user client PC 7 automatically accesses the mail server PC 6 or the central mail server 5 to receive a mail service (see Fig. 4). If the mail server PC 6 can provide a mail service, the user client. PC 7 accesses the mail server PC 6 using the network access information, downloads incoming mail, which has been received at a mail account of the user, and displays the downloaded mail on the screen so that the user can check the mail. In the present invention, 24 hour operation of the mail server PC 6 cannot be ensured because the mail server PC 6 is a general-purpose PC rather than expensive, dedicated server equipment and because the user of the mail server PC 6 uses it for their personal work as users of general client PCs do. In the dual mail service system 1 according to the present invention, the user client PC 7 can always receive a mail service by requesting that the central mail server 5 provide a mail sending/receiving service even when the mail server PC 6 cannot provide a mail service due to network failure, mail server loading, power-off, etc., upon receiving a mail sending/receiving service request from the user client PC 7. Marks "O" on arrows shown in Figs. 2, 4, 6a, and 6b indicate that the mail server PC 6 can provide a mail service normally, and marks "X" on arrows shown in Figs. 2, 4, 6a, and 6b indicate that the mail server PC 6 cannot provide a mail service. Fig. 3 illustrates the configuration of an integrated central mail server 200 according to an embodiment of the present invention. A receive-only SMTP server 3, a send-only SMTP server 4, and all central mail servers 5, which constitute a dual mail server system 1, can be implemented as one integrated central mail server 200. Distributed and integrated environments have their own merits and drawbacks. The integrated environment is suitable when it is easy to manage mail accounts because there are a small number of users having mail accounts, and also has an advantage in that it can reduce transmission/reception load over a network. However, the integrated environment has a disadvantage in that all mail services may be terminated when the integrated central mail server 5 is inoperable since management of all mail services is carried out under the control of the single server 5. On the other hand, the distributed environment is advantageous in terms of resource management since it reduces server load when an excessive number of users is present, and also has an advantage in that, even when a specific server fails, the remaining servers can provide stable mail services normally. However, such high efficiency and stability comes at high cost due to additional equipment. Fig. 4 illustrates an automatic access function of the user client PC 7 according to the present invention. To implement the automatic access function according to the present invention, the central mail server 5 manages a domain name of a mail server PC 6 which the client PC 7 specifies as the mail server. Specifically, the central mail server 5 serves as a domain name server which provides an actual physical mail server address corresponding to the domain name of the mail server PC 6. When the user client PC 7 intends to access the mail server PC 6 for sending/receiving mail, the user client PC 7 transmits the domain name of the mail server PC 6 to the central mail server 5. Then, the central mail server 5 transmits network access information of the mail server PC 6 if the mail server PC 6 can provide a mail service, and transmits network access information of the central mail server 5 if the mail server PC 6 cannot provide a mail service. Here, the term "network access information" refers to an actual physical address which includes an IP address and a path. Thus, it is preferable that the central mail server 5 determine, in real time, whether or not the mail server PC 6 can provide a mail service, and it is necessary to associate network access information of the mail server PC 6 or the central mail server 5 with the domain name of the mail server PC 6 according to the real time determination. It is also necessary that the mail server PC 6, which has been restored from an unavailable state to an available state, download information of a mail service, which the central mail server 5 has provided to the client PC 7 while the mail server PC 6 was unavailable, from the central mail server 5, and update its mail information with the downloaded mail information. Fig. 5 is a detailed block diagram illustrating a dual mail service system 1 according to an embodiment of the present invention. A detailed structure of the dual mail server system 1 will now be described with reference to Fig. 5, in conjunction with a description of mail sending/receiving procedures in the dual mail server system 1. First, the mail receiving procedure is described. If a sender specifies, as a recipient, a user who has a mail account in the dual mail service system 1 according to the present invention, and sends mail to the recipient, a receive-only SMTP server 3 receives the mail from an external SMTP server 2 corresponding to the sender. The receive-only SMTP server 3 according to the present invention comprises an incoming mail receiver 31, a recipient address searcher 32, an incoming mail transfer unit 33, and a main DB 321. If the incoming mail receiver 31 receives incoming mail, the recipient address searcher 32 searches the main DB 321 for a central mail server 5, to which the mail account of the recipient belongs, using a mail address of the recipient (for example, aaa@xxx.com). The incoming mail transfer unit 33 transfers the incoming mail to the central mail server 5 found by the recipient address searcher 32. The central mail server 5 according to the present invention comprises an incoming mail receiver 51, an incoming mail manager 52, an incoming mail information DB 521, an incoming mail buffer 522, an outgoing mail transfer unit 53, a destination address searcher 54, an outgoing mail receiver 55, a network access information manager 56, and a network access information DB 561. The incoming mail manager 52 stores the incoming mail received through the incoming mail receiver 51 in a user home directory of a user of a mail server PC 6 corresponding to the recipient mail account. Information of the incoming mail, such as a recipient mail account, a title, a sender mail account, and a sending date, is recorded in an incoming mail information DB 521 of the user home directory. A message body file of the mail is stored in an incoming mail buffer 522 implemented in the user home directory. The incoming mail buffer 522 buffers recently received incoming files within a set storage capacity limit. The incoming mail manager 52 also searches the incoming mail buffer 522 to select recent incoming mail stored therein, and transmits the selected mail upon receiving an incoming mail transmission request from the mail server PC 6 or from the user client PC 7. For all mail server PCs 6 registered in the central mail server 5, the network access information manager 56 checks, in real time, whether or not each mail server PC 6 can provide a mail service, and sets its network access information in the network access information DB 561. Two methods may be employed to check, in real time, whether or not the mail server PC 6 can provide a mail service. In the first method, the network access information manager 56 transmits a mail service availability check request to the mail server PC 6, and the mail server PC 6 responds with information indicating whether or not the mail server PC 6 is operable as a mail server. If the central mail server 5 receives no response or a response indicating that the mail server PC 6 is inoperable, the central mail server 5 determines that the mail server PC 6 cannot provide a mail service. In the second method, when the mail server PC 6 can provide a mail service, the mail server PC 6 periodically pushes a "service available" indication signal to the network access information manager 56 of the central mail server 5. If the central mail server 5 does not receive the "service available" indication signal within a limited time, the central mail server 5 determines that the mail server PC 6 cannot provide a mail service. The mail server PC 5 according to the present invention comprises an incoming mail receiver 61, an incoming mail transmitter 62, an outgoing mail transfer unit 63, and an outgoing mail receiver 64. The mail server PC 6 is a general PC in which a mail server program has been installed. The mail server PC 6 preferably has a web server function. Users can use the mail server and can also receive a mail account service therefrom. The incoming mail receiver 61 receives incoming mail from the central mail server 5. Upon receiving a transmission request from the user client PC 7, the incoming mail transmitter 62 transmits the received incoming mail to the user client PC 7, and then the user checks their own incoming mail using the client PC 7. Specifically, if the user activates a mail client program and selects a "get mail" function, the user client PC 7 transmits a domain name of the mail server PC 6 to the central mail server 5. The network access information manager 56 in the central mail server 5 searches the network access information DB 561 for network access information corresponding to the domain name received from the user client PC 7, and transmits the found network access information back to the user client PC 7. If the mail server PC 6 can provide a mail service, the user client PC 7 receives a path to access the mail server PC 6 as network access information, and, if the mail server PC 6 cannot provide a mail service, the user client PC 7 receives a path to access the central mail server 5 as network access information. Thus, the user client PC 7 automatically and selectively accesses the mail server PC 6 or the central mail server 5 according to network access information set by the central mail server 5 to receive incoming mail. Next, a description will be given of a mail sending procedure in the dual mail server system 1. If the user composes outgoing mail using a mail client program installed in the user client PC 7 and selects a sending function, the user client PC 7 transmits a domain name of a mail server PC 6, specified as the mail server, to the central mail server 5, and receives network access information from the central mail server 5. This network access information is URL/path information, containing an IP address, of the mail server PC 6 or the central mail server 5 according to whether or not the mail server PC 6 can provide a mail service, as described above in the mail receiving procedure. If it is assumed that the user client PC 7 has received the network access information of the mail server PC 6, the outgoing mail receiver 64 in the mail server PC 6 receives the outgoing mail from the user client PC 7, and the outgoing mail transfer unit 63 transfers the outgoing mail to the central mail server 5. On the contrary, if it is assumed that the user client PC 7 has received the network access information of the central mail server 5, the outgoing mail receiver 55 in the central mail server 5 receives the outgoing mail directly from the user client PC 7. The destination address searcher 54 in the central mail server 5 searches for a recipient mail address of the outgoing mail, and determines where to send the outgoing mail. According to the determination, the outgoing mail transfer unit 53 leaves the outgoing mail in the same central mail server, transfers the outgoing mail to a different central mail server, or transfers the outgoing mail to the send-only SMTP server 4. If the outgoing mail is left in the same central mail server, it is also determined whether the outgoing mail is transmitted to the same mail server PC or a different mail server PC. If the outgoing mail is to be transmitted externally, an outgoing mail receiver 42 in the send-only SMTP server 4 receives the outgoing mail from the central mail server 5. An outgoing mail sender 41 in the send-only SMTP server 4 sends the outgoing mail to an external SMTP server 2 corresponding to the recipient of the outgoing mail. Figs. 6a and 6b illustrate mail transmitting/receiving environments in the dual mail service system 1 according to an embodiment of the present invention. The dual mail service system 1. according to the present invention allows the client PC to receive both a web mail service through a server web site and a P0P3 mail service through a mail client program of the client PC. Fig. 6a illustrates a web mail service environment. The web mail service has an advantage in that, at any PC, each mail account user can log into the web site of a mail server PC and can send/receive their mail through a web interface. Domain name information of the mail server PC6, which the user has entered into an address bar in a web browser, permits access to the web site of the mail server PC 6 or to the web site of the central mail server 5 depending on the network access information provided by the central mail server 5. Accordingly, if the mail service through the web site of the mail server PC 6 is not available for whatever reason, the user client PC 7 logs into the website of the central mail server 5 to send/receive their mail. Fig. 6b illustrates a mail service in a POP3 server environment. The user sets, in a mail client program, their mail account access information such as a mail address and a password, a POP3 server information, and an SMTP server information, and sends/receives mail using the mail program. The POP3 and SMTP server information set in a mail client program includes the domain name of the mail server PC 6. The POP3 server environment has an advantage in that the user can easily send/receive mail through a mail program without the need to enter an ID and password each time they log into the mail server PC 6. The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

INDUSTRIAL APPLICABILITY A system for providing a dual mail service using a mail server PC and a central mail server according to the present invention has the following advantages. A mail server PC, which is a general PC with a mail server implemented therein, can provide a mail service to a number of mail users. When the mail server PC cannot provide a mail service, the central mail server allocates access information of the central mail server to the domain name of the mail server PC, and provides the access information of the central mail server to a user client PC, so that the user client PC can receive a dual mail service through the mail server PC or the central mail server. The mail server PC can operate as a general PC while performing a mail server function. Thus, the user may power on the mail server PC only when using it for their work. Even if the user powers off the mail server PC when not using it, the central mail server substitutes for the mail server PC and provides the mail server function. Thus, mail account users can constantly receive a mail service without inconvenience even when the mail server PC performs the mail server function and the general client PC function in a parallel fashion.