There are three important objects in the mailing system: the mailer
object itself, the mail daemon, and the mail command.

Mail messages are stored in a mapping with six fields: subject, to,
from, text, unread, and deleted. Subject is the subject of the message.
To is the list of users to whom the mail was sent. From is the user
who send the message. These three fields are all strings. Text is a
single string which is the text of the message. It contains line feeds
if the mail message has more than one line. The unread and deleted
fields are integers which are 1 if the mail is unread or has been
deleted by the user, respectively, and 0 if not. Each user has a mail
spool which is an array of these mappings. When the user exits mail,
or when a new mail message is added to his mail spool, this array is
saved to disk in /data/mail with the name of the user as the file name.

The mailer is the most complicated of the three. When the user enters
mail, a clone of this object is loaded. It will have object privileges
and its creator will be the name of the player who entered the mail
command. First create() is called, then the mail command call the
initialize() function. If the player typed "mail foo" then initialize()
will begin sending a mail message to foo. Otherwise it will input_to()
the function get_mail_command() to enter the main loop of the mailer.
The users mail spool is restored by create() if it exists, otherwise
a blank mail structure is initialized.
The function get_mail_command() takes a string of input from the user and
tries to parse it as a mailer command. A blank string will display the
current message. A number will display that number message. After that,
the code checks the first character of the string against various commands.
"m" sends a new mail. "r" replies to a message. "d" deletes and "u"
undeletes mail. "q" quits from the mailer. "h" prints the user's
mail headers. "?" prints a couple of lines of help. There is one
function (or more in some cases) to handle these commands.
Reading mail makes a call to the more_strings() function in player.c
to more the file. Sending mail calls begin_edit() in the player.c to
edit the message and calls the mailer daemon (see below) to send the
mail. At the end of each function (except quit) the mail prompt is
written and input is directed to get_mail_command() to handle the
next command. When the user quits, his spool is saved and the mailer
object destructs itself. (Note that the mailer object is not moved
into the player's inventory, unlike the traditional method.)

The mail daemon defines two functions. One, add_mail_to_spool(), takes
the name of a user and a mail message. It restores the user's spool,
adds the new message, saves the spool, erases its internal memory, and
returns. This is necessary because the mailer only has permissions to
read and save the spool of the user that loaded it: the daemon has the
power to write to any mail spool, but it can only be used to add new
messages, not delete messages or change their contents.
The second function, count_unread_mail(), restores a mail spool for a
named user, counts the number of unread messages in the spool, erases
its memory, and returns the number of unread messages. It's used at
login time to tell the user if he has new mail. It could also be added
to the finger command but that has not been implemented.

The mailer command is in /cmds/player, which means that players can
read mail from anywhere in the mud. You may want to move this command
to /cmds/wizard and add a post office object which does what the
command does, but only when a player is in the post office. All it
does is clone a mailer object for the player and call the initialize
function in it.