Data storing method used for data store server in mailing system

A mail server for storing in a data store server mails received from a mail sending apparatus, wherein the mail server includes,

• at every domain of respective destination addresses of the mail, sequence management information for each domain which manages (at least) a sequence of stored mails and transmission time when the mail server sends the stored mails to a destination server,
• wherein the sequence management information for each domain is used to manage data size and number of mails managed thereby, and includes a plurality of lower rank sequence management information, each of which is used to manage each entry defined as a combination of a key corresponding to the mail as a value thereto, and the transmission time,
• wherein each of the plurality of lower rank sequence management information is used to manage data size and number of mails managed thereby, and
• wherein the mail server prepares each of the plurality of lower rank sequence management information so as to satisfy conditions that the number of entries in the each lower rank sequence management information is made not greater than a predetermined number, and that a difference between earliest and latest sending times of mails included in the entries contained within the each lower rank sequence management information is made not greater than a predetermined value,
• wherein the mail server manages the plurality of lower rank sequence management information in a ranked manner, and
• wherein the mail server updates the data size and the number of mails managed by the sequence management information for each domain by using the data size and the number of mails managed by each of the plurality of lower rank sequence management information.