Keeping an eye on your mail system is about more than making sure the mail gets through. You should also be thinking about spam statistics, usage of the mail system by users and whether users are really getting the best out of the systems that they provide. In Systems Administration Toolkit: Monitoring mail usage that’s exactly what I cover:
Examine methods for reporting on e-mail usage and getting detailed statistics on mailbox use. E-mail logs can provide a wealth of information about your users and their e-mail habits, including how often and frequently they use their e-mail. Depending on your UNIX systems, you might even be able to glean more detailed information, such as identifying which users are using their folders effectively and who could benefit from further training and advice.