As business activities becoming more digitalized, electronic information is often produced as vital evidence during civil litigation. The process of discovering information as evidence is getting increasingly expensive as the volume of data explodes. This surging demand calls for a solution to reduce the cost associated with discovery. In this paper, we propose filtering included emails as a means to reduce the volume of review data, and describe efficient algorithms to identify those emails. Experiments show that this can reduce the number of emails to be reviewed by 20% in a corporate email corpse.