I'm not sure that involving a Java VM at all is the most efficient strategy. If I had the time (which I unfortunately don't...) I'd look at porting the MIME functionality to TCL, possibly using tcllib [which may itself have to be ported from TCL 8.x], and then running the MIME code right inside AOLServer. aD is probably using Java because it can run inside Oracle in a relatively efficient and conventient manner. That's not the case with Postgres.
As a distant second, I'd suggest looking into using the JVM as a server, handling each request from OpenACS within the same process. But if you're going to do that much coding, why not implement the mail handling server in something like Python? There's already server frameworks, MIME parsers, etc.
