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.