1. A computer implemented e-mail analysis system comprising: a keyword database that retains a plurality of keywords, each of the keywords having a correspondence with one or more classification codes, the classification codes being used to classify one or more e-mails; a processor; and a memory, wherein the memory has stored thereon instructions that, when executed by the processor, causes the processor to: search one of the keywords in a first e-mail and assign a first classification code corresponding to the keyword; accept a second classification code assigned by a user for a second e-mail to which no classification code has been assigned; calculate a score, which indicates strength of linkage between the first e-mail and the first classification code as well as between the second e-mail and the second classification code, and quantifies priority of the e-mails to be monitored, on the basis of a plurality of terms that respectively appear in the first e-mail and the second e-mail to which the one or more classification codes have been assigned and an evaluation value of each of the plurality of terms; learn the evaluation value of each of the plurality of terms based on a process of classifying the one or more e-mails, the learned evaluation value being used to calculate the score; prepare an investigation report by using the calculated score when the calculated score exceeds a specified threshold value arbitrarily set by the system on the basis of an input from the user; and promote efficiency in monitoring the e-mails by feeding back the score to the user by outputting the prepared investigation report.