For one thing Spamassassin is quite hopeless compared to Gmail's filter (and yes I have compared). But more to the point I would absolutely equate web with server-side hosted app. For example, if you want to implement decent search it's probably going to be Lucene-based. So if you are just talking thin-client presentation layer, the app isn't really native in any case - the acid test for a non-web app is whether it works without an internet connection. (Other major differences would be ubiquity and maintenance.)