From Matt Chant:
There's an unsupported way, which is exactly the same as the unsupported way in the MTA . The only difference is that the database is now Mail.Box and not MTAForms.nsf.
The problem is that this is only likely to work if messages are in CD format and being converted to MIME on that particular system. As you migrate to R5 and the client is now submitting the MIME, then this won't work.
There is an outstanding enhancement request for this feature. It's not as simple as it might sound, as there are pluses and minuses to doing on a server (Hard to inject additional content into existing MIME, Can't do it for S/MIME at all )and pluses and minuses to doing on the client (Solves the MIME,S/MIME problems but then there are problems in trying to support disclaimers for only external recipients.), so it's going to take some figuring out how to achieve it cleanly in the modern Domino environment.