This page about e-mail disclaimer is quite old but remains up to date. Today, all major companies attach disclaimers to their outgoing e-mails. Usually, nobody takes time to read them. You should! Some of them are really funny to not say stupid.
Most of the time, disclaimers have a legal character: Even if it will not protect you, it can have a positive impact in case of prosecution. It can be seen as a first step in the process of due diligence.
However, a good e-mail disclaimer does not resume to asking the local system administrator to configure the outgoing mail relay to automatically attach a piece of text to any outgoing message. A disclaimer will never protect the company against data loss for example. Instead, the disclaimer must be part of an internal e-mail policy. And, once again, the end-users must be aware of risks they face using e-mail applications (via continuous awareness trainings).
If you really need to use a disclaimer, try to keep it concise! A mail of two lines with twenty lines of disclaimer is really irritating. Why not refer to an optional HTML page (a good example here) with useful information and links to procedure (ex: how to report a problem). Avoid direct HTML in disclaimers.
Finally, what’s the best way to manage e-mail disclaimers? Add them on top or bottom of a message? (visibility will be much better on top). Allow the user to include the disclaimer or not? Add the disclaimer based on the e-mail content? (based on its classification?).