Whoson ("WHO iS ONline") is a proposed Internet protocol that allows Internet server programs know if a particular (dynamically allocated) IP address is currently allocated to a known (trusted) user and, optionally, the identity of the said user.
The protocol could be used by an SMTP Message Transfer System in conjunction with anti-spam-relaying filters to implement a scheme similar to the one described here to allow roaming customers use their "home" SMTP server to submit email while connected from a "foreign" network.
Look here for the draft specification (unpublished RFC). And here is the same document in the source (nroff -ms) format.
Prototype implementation of the protocol is avaiable from the:
Eugene Crosser <crosser at average dot org>