This　paper　proposes　a　simulation　model　to　characterize　virus　propagation　in　an　email　network　and　analyzes　various　defense　strategies.　We　demonstrate　the　key　factor　which　affects　the　efficiency　of　different　strategies,　and　illustrate　two　phases　of　virus　propagation.　The　results　show　that　D-steps　immunization　is　a　feasible　strategy　in　the　case　of　finite　resources　and　the　efficiency　of　node-betweenness　immunization　is　better　than　targeted　immunization　if　we　have　the　global　information　about　a　network.　©　2009　Springer.