2. The client computer system of claim 1 , wherein the at least one processor is further configured to: send the text fingerprint to a server computer system; and receive from the server computer system a target label determined for the target electronic document, the target label indicative of a category of documents that the target electronic document belongs to, wherein determining the target label comprises: retrieving a reference fingerprint from a database of reference fingerprints, the reference fingerprint determined for a reference electronic document belonging to the category, the reference fingerprint selected according to a length of the reference fingerprint so that the length of the reference fingerprint is between the upper and lower bounds; and determining whether the target electronic document belongs to the category according to a result of comparing the text fingerprint to the reference fingerprint.
3. The client computer system of claim 2 , wherein the category of documents is a spam category.