'Add a message to the mailbox.
@type txt: L{bytes} or file-like object
@param txt: A message to add.
@rtype: L{Deferred <defer.Deferred>}
@return: A deferred which fires when the message has been added to
the mailbox.'

def appendMessage(self, txt):

    task = self.AppendFactory(self, txt)
    result = task.defer
    task.startUp()
    return result
