Incremental anti-spam lookup and update service

The present invention provides a unique system and method that facilitates incrementally updating spam filters in near real time or real time. Incremental updates can be generated in part by difference learning. Difference learning involves training a new spam filter based on new data and then looking for the differences between the new spam filter and the existing spam filter. Differences can be determined at least in part by comparing the absolute values of parameter changes (weight changes of a feature between the two filters). Other factors such as frequency of parameters can be employed as well. In addition, available updates with respect to particular features or messages can be looked up using one or more lookup tables or databases. When incremental and/or feature-specific updates are available, they can be downloaded such as by a client for example. Incremental updates can be automatically provided or can be provided by request according to client or server preferences.