PM Lite is a lightweight private messaging module that defines a new PM content type and creates PMs as nodes. The main idea behind this module is that private messages represent content on a site with extremely limited access. This module defines the private message node type and adds a recipient field to its creation form where users can enter single or multiple recipients. Once created, private messages may not be modified and can only be viewed by the author and the recipients.
The goal of the module is to remain as small as possible. By creating private messages as nodes, if you need functionality beyond what the module provides, you can build it yourself using any number of contributed modules that were designed to work with Drupal's node system.