This event is fired when client sends 'RSET'
command, thus canceling any previous commands.
For example, client may send MAIL FROM: and RCPT TO:
commands (and you will receive appropriate events for those
commands) and after that it can 'change it's
mind' and reset it - to start all over (with new sender
and recipients). Since you must know that any previously
stored recipients are now invalid - you should update that
information from this event.
RSET command can be issued anytime during message
posting - except when actual body is transmitted.