Sendmail has a new white paper available (PDF) on the impact of machine-generated messages on enterprise email infrastructure. The paper describes the category of machine-generated messages, the challenges they create, and what you can do to meet those challenges. It also offers examples of applications/devices which generate mail so you can identify them in your organization. It is especially useful for those looking to get better control over mail flow and for those planning or executing a migration to the cloud.
I would also recommend two related papers: