A "mail recipient" is, by definition, an object that can be sent mail.
Mail recipients must either be players or descendants of $mail_recipient.
One source of confusion is that the terms "mail recipient", "mail folder", "mailing list", and "mail collection" really all refer to the same kind of object. It so happens that $mail_recipient serve several distinct functions and we tend to use whatever term happens to best match the application under discussion, e.g., it's a "mailing list" if we're playing with its .mail_forward property but it's also a "mail folder" if we're examining the messages that have been saved in it.
Note that, by default, a freshly created recipient is accessibly only by you. If you wish to make a publically accessible recipient, set .readers=1. Furthermore, if you want to allow a message on your recipient to be removed by its sender without your intervention, set .rmm_own_msgs=1. Finally, in order for other players to be able to refer to your recipient by name, the object must reside in $mail_agent. $mail_agent will not accept the object unless it has an actual description and a name distinct from all other mail recipient names/aliases.
Topics:
MR-access -- controlling read, write and send access to a recipient MR-naming -- naming conventions and how to match on recipient names MR-sequences -- message sequence arguments to $mail_recipient verbs MR-reading -- reading messages/headers on recipients MR-searching -- searching message lists for patterns in certain fields MR-writing -- removing and renumbering messages MR-subscribing -- updating .mail_forward, .mail_notify
and the story of .current_message MR-expiration -- expiring and netmailing messages from recipients