1. A community-based collaborative system connected to a client terminal via a network and supports knowledge accumulation by categorizing and accumulating messages posted from the client terminal to a plurality of virtual communities, the plurality of virtual communities comprising at least a first virtual community and a second virtual community, the system comprising: access control means for making user authentication of the client terminal so as to permit the client terminal to post a message; message subscription type setting means for setting a first subscription type for the first virtual community, the first subscription type indicating a way a user subscribes to the messages categorized and accumulated in the plurality of virtual communities in accordance with an instruction from the user; community processing means for managing the plurality virtual communities, and categorizing and accumulating messages posted, to the plurality of virtual communities, from the client terminal, the client terminal being granted access permission for specified topics by said access control means, said community processing means including: reply message processing means for, when a reply email message used to post a reply message is received from the client terminal in response to an email message comprising a header that includes an ID, subject that includes a community name and an assigned number, and a message text of a new posting on a virtual community, determining whether or not a header of the reply email message includes the ID, categorizing and accumulating the reply message based on whether the header of the reply email message includes the ID, and delivering the categorized and accumulated reply message to a subscriber; and a processor for implementing at least the message subscription type setting means, wherein: the ID comprises a message ID and a thread ID, and the reply message processing means categorizes and accumulates the reply message based on the message ID and the thread ID if it is determined that the header of the reply email message includes the message ID and the thread ID, and based on the community name and the assigned number included in a subject of the reply email message if it is determined that the header of the reply email message does not include the message ID and the thread ID.