1. A method, implemented on server having one or more processors and memory storing programs to be executed by the one or more processors, for email classification in a spam-filtering system, comprising: storing email message data and associated original classification data in a data cache, wherein the original classification is one of a spam classification and a trusted classification with respect to a first email message; receiving external evidence after the original classification of the stored email message data, wherein receiving external evidence includes receiving one or a combination of new keywords and new return-path features extracted from email message bodies; and responsive to receiving the external evidence, launching a re-evaluation of the first email message stored in the data cache based on the external evidence, wherein the re-evaluation includes one of validating and changing the associated original classification of the first email message, and wherein the first email message is re-evaluated based on the external evidence and one or more variables used to determine the original classification of the first email message.