1. A method of detecting whether a first e-mail is undesirable, the method comprising: inputting the first e-mail to each of a plurality of constituent spam classifiers; obtaining at least one score from each of the plurality of constituent spam classifiers indicating the degree to which the first e-mail is deemed spam; obtaining a combined spam score from a combined spam classifier that takes as input the at least one score from the plurality of constituent spam classifiers, the combined spam classifier being computed automatically in accordance with a false-positive vs. false-negative tradeoff; and identifying the first e-mail as an undesirable e-mail if the combined spam score indicates that the first e-mail is undesirable; wherein step of computing the combined spam classifier comprises: compiling a labeled e-mail corpus consisting of a plurality of e-mails that have been labeled according to the degree to which the plurality of e-mails are deemed to be spam; computing scores of the plurality of constituent spam classifiers on each e-mail in the labeled e-mail corpus; establishing a set of one or more sample false-positive vs. false-negative tradeoffs; analyzing, for each sample false-positive vs. false-negative tradeoff, the computed scores of the plurality of constituent spam classifiers on each e-mail in the labeled e-mail corpus to compute a set of combined spam classifiers, each of which best achieves a corresponding sample false-positive vs. false-negative tradeoff; selecting a false-positive vs. false-negative tradeoff; and computing from the false-positive vs. false-negative tradeoff, a set of sample false-positive vs. false-negative tradeoffs and a set of corresponding best combined classifiers a best combined classifier for the false-positive vs. false-negative tradeoff, and wherein the false-positive vs. false-negative tradeoffs are specified by penalty functions, and the combined spam classifier associated with a given penalty function is computed by an optimization procedure that yields the combined spam classifier for which the value of the given penalty function is minimal on the labeled e-mail corpus.