The correct answer would be to set up signature blocks for each user that include the disclaimer and include it for all e-mail leaving and also have a company policy requiring the disclaimer. This would automate the insertion of the disclaimer in every outgoing e-mail.
In addition, there is no guarantee that the signature block will be included for all outgoing messages. Users could certainly delete or alter it if left in their control. Generally, when an organization is looking into disclaimers, they want something that is forcibly appended to all outgoing messages (e.g., something that is enforceable). Signature blocks are not enforceable, and can be changed, deleted and even turned off.
So while the signature block solution is an interesting approach, IMHO, I don't think it is what the original poster was looking for.
This was first published in June 2003