I have no experience with Exchange. However, I found the following website:

http://www.exclaimer.co.uk/

One of the features is the following:

Disclaimers
Enabling this feature allows text and images to be added to all or selected mail. This process occurs when the mail reaches the mailserver after having left the mail client software. This means that the disclaimer can be attached to mail being sent or received by any software configured to use the mailserver on which eXclaimer is installed. This includes Microsoft’s Outlook Web Access and any mail relayed through this server.

Yet another piece of software which handles disclaimers being added to all outgoing e-mails is called MailEssentials. Info about it is here: http://www.sofsol.co.nz/p0002123.htm