'Return the path to a user\'s mail directory.
@type name: L{bytes}
@param name: A username.
@rtype: L{bytes} or L{None}
@return: The path to the user\'s mail directory for a valid user. For
an invalid user, the path to the postmaster\'s mailbox if bounces
are redirected there. Otherwise, L{None}.'

def userDirectory(self, name):

    if (name not in self.dbm):
        if (not self.postmaster):
            return None
        name = 'postmaster'
    dir = os.path.join(self.root, name)
    if (not os.path.exists(dir)):
        initializeMaildir(dir)
    return dir
