Another possibility is to make each message a file; my mail system on
the Acorn does this. I has costs on some platforms I imagine, most seem
to have a minimum file size of some sort so you can waste space easily.
With disk prices as they are these days, I don'timagine that is a
particularly worrisome mater.
It does offer an interesting sort of security in that it is quite
difficult to end up deleting parts of messages and there is something
vaguely transactional about it. If the index file(s) stored the relative
paths of the messages then that would presuambly map to message IDs
reasonably. It might even help with avoiding the redundant loading of
message left on the server.

Now of course if we had machines able to use the file system we designed
at Interval, indexable by content and tags etc, this would all be much
simpler. Sigh.

tim
-- 
Tim Rowledge, tim at sumeru.stanford.edu, http://sumeru.stanford.edu/tim
Supercomputer:  Turns CPU-bound problem into I/O-bound problem.  - Ken Batcher