JBM == JBMS (JBM actually refers to JBoss Messaging the rewrite of JBossMQ)
The message should be routed via the mail listener to a special mailbox or queue.
I can see this done like this. The MailListener removes the mail body and created a new body with "BLA BLA would you like it delivered message pertaining to 'buy viagra extra cheap' if so then click here".. Also the original headers are written to a new store object.
A handler servlet (on the mail server no doubt) triggers an event that takes the original header, constructs a mail object, adds the header and reinvokes down the listener chain. The trick is that we don't have "envelopes" yet. Basically additioanl headers that cannot be written from outside. As an intermediate klude a special header could be used that contains the mail's body number (because outsiders couldn't guess this very easily) --- but I'd far rather see us implement envelopes.
In short, no, please impelemnt this though becasue its on the list of things to do anyhow.