Abstract: A receiver is shielded from undesirable electronic messages, sometimes referred to as “spam”, by requiring any sender who wants to send an electronic message to the receiver to first submit to a filter. The filter is specified by the receiver. The receiver-specified filter can be downloaded to the sender's computer and executed on the sender's computer. By execution, the filter elicits information from the sender and uses it without disclosing it to authorize, or not, the sending of electronic messages (such as email) from the sender to the receiver. Another embodiment according to the invention allows a sender to prevent undesirable receivers from viewing the sender-sent electronic messages by requiring any receiver who wants to view an electronic message from an authorized sender to first submit to a filter. One or more third parties can be involved.