In a main embodiment, prestage filtering is applied via user-definable filter parameters (e.g., reject, pass, or granularity filters) on data being transferred between a communication unit (201) and communication server (220). For downloading, e.g., email from a host post office (240), a communication server controller (229) preferably either forwards the filter parameters in a query object or message to the post office to apply and return qualified mail (406-414), or the communication server receives all unprocessed mail and applies the filters locally (418-420), only acknowledging as processed that mail which passes the filters. For uploading, e.g., email from a client, a client controller applies an upload prestage filter (432) so as to retain all filter-rejected email, while transmitting email passing the filters (434). Thus, only desired data transfers (i.e., those meeting user defined filters) are communicated over the expense-bearing networks between the remote unit and communication server.