1. A computer-implemented method for analyzing an email message to determine the likelihood that the email message is spam, the method comprising: (a) for each of a plurality of message attributes, determining a message attribute occurrence frequency within a collection of known spam email messages, the message attribute occurrence frequency specifying how often one of the plurality of message attributes occurs in the collection of known spam email messages; (b) for each of the plurality of message attributes, determining a message attribute occurrence frequency within a collection of known non-spam email messages; (c) determining a spam probability weight for each of the plurality of message attributes based at least in part on the determined message attribute occurrence frequencies determined at steps (a) and (b), wherein step (c) comprises, when determining the spam probability weight for each of the plurality of message attributes, (c.1) weighting more highly newer email messages within the collection of known spam email messages than older email messages within the collection of known spam email messages, and (c.2) weighting more highly newer email messages within the collection of known non-spam email message than older email messages within the collection of known non-spam email messages, wherein the weighting more highly newer email messages within the collection of known non-spam email messages at (c.2) is to a lesser extent than the weighting more highly newer email messages within the collection of known spam messages at (c.1): (d) detecting a further email message; (e) identifying which, if any, of the message attributes are associated with the further email message; and (f) determining a value indicative of a likelihood that the further email message is spam based at least in part on the message attributes identified at step (e) as being associated with the further email message and spam probability weights corresponding to the identified message attributes; wherein at least step (f) is performed using a hardware processor.