Players can send mail to the player controlling an entity by mailing to entity@game.pbm.com.
For example, suppose you saw the following characters:
Seen here: Circe [2225], with three peasants, accompanied by: Hephaestus [3446], with 11 peasants
Circe's lord isn't identified, so you don't know what player is controlling her. However, it is still possible to send Circe's owner a message. Mail to 2225@g1.pbm.com and the message will be forwarded to whatever player is controlling Circe [2225].
Forwarding works for character and player entities. Mail sent to other entity numbers, or to unaffiliated characters, will be silently discarded.
Forwarded mail is not anonymous. The original headers on your message will be preserved.