/mail read - read your mail
/mail send <playername> <message> - send mail to a player
/mail sendall <message> - send mail to ALL players - don't spam
/mail clear - clear all of your mail

/msg <playername> <message> - send an instant message to a player

/r <message> - reply to the last message you received

If you would like to form a group/clan with special prefixes in chat, contact an admin.