listener :
  - runs and listens for connections
  
  - on getting one, it fires up a session. to deal with the responses
  
session : ( Imapd)

  - dispatch each operation to an operator.
  - deals with information about server
  - deals with information about current mail store open
  - deals with authentication state..
  getSelectedMailbox();  - selectedMailbox;
  close(Mailbox mbox) - rely's message to mbox, then sets selectedMailbox to null;
  selectMailbox(char[] name) 
     // asks depot to getMailbox()
     // set selected mailbox to it.
     // returns the selected Mailbox.
  mailboxReadOnly = true/false - 
  
operator:
  - needs to rely on session for state information
  - needs to rely on session for current mailstore information
  
  
  
depot - global manager of open mailstores
  Mailbox[] openMailboxes;
  getMailbox(session, name);
  


Mailbox  (eg. an INBOX)
   referenceCount; // number of instances that are using it..
   close() = reduces refernceCount, removes it from depot if down to 0
    
   int firstUnseen // first unseen message
   int uidValidity() //????
   int uidNext()
   
Message = representation of a single message.