send someone a mail
Without arguments, this command will put you into mail client to read and manage your mail. This client is coded to behave much like standard mail on unix systems. The internal help is accessed with the ? command, or with <help mail_syntax>.
The command mail with the argument NAME will put you into an editor where you can type mail for another player NAME. A recognized name won't work for this purpose, so you must know the name a player used for character creation.
This form of communication is considered to be Out Of Character, between players, and not between characters.