The Easiest and most obvious way is to use either the VBScript or VB method that Microsoft provides, But for the non coders out there (mmm?) your only option is to use a third-party tool to add a disclaimer to outgoing Exchange messages.

Some reasonably priced ones are available, although No matter which solution you use, you'll have to be careful about where and how you set it up. The easiest way to prevent disclaimers from appearing on your internal messages is to set up whatever disclaimer tool you plan to use on your external bridgehead, not on your internal servers.

You can set up a separate SMTP Virtual Server for outbound-routed traffic, then apply the disclaimer to messages passing through the server.  As for effective 3rd party tools here is couple of relatively low-priced disclaimer software products (less than $100US) are QS Disclaimer and DisclaimIT.