Abstract: Systems, methods, and software that implement a challenge protocol to qualify e-mail senders before delivery of e-mail messages. Preferably the challenge protocol is implemented to create minimal burden on human senders, but a significant burden on bulk e-mail programs such that an active disincentive to bulk e-mail practices is provided. Messages from an unrecognized sender are quarantined until the message-designated sender complies with the challenge protocol. Once a sender has complied with the challenge protocol, the sender is included in an inclusion list maintained for the message-designated recipient ID. E-mail messages from senders on the inclusion list can bypass the challenge protocol and be forwarded to the message-designated recipient ID. In particular implementations, periodically or on request, a digest of quarantined messages can be provided to the corresponding recipient ID to allow users an opportunity to manually augment the inclusion list.