Abstract: An apparatus and method for classifying e-mails using a decision tree is disclosed. The apparatus includes: a client e-mail storing unit for storing e-mails according to each folder; a decision tree generating unit for generating a decision tree based on information about e-mails stored according to folders in the client e-mail storing unit; a received e-mail processing unit for receiving e-mails; an e-mail storing unit for storing the e-mails received in the receiving e-mail processing unit; an e-mail classifying unit for classifying the e-mails stored in the e-mail storing unit based on the decision tree; an e-mail transmitting unit for transmitting the classified e-mail to the client e-mail storing unit; and a controlling unit for controlling the units to generate the decision tree based on the e-mails stored in the client e-mail storing unit and to classify the e-mail transmitted from outside based on the decision tree.