John Hardin suggested to Brad Wyman (and Jignesh Ruparel just posted a
question equivalent to Brad's),
| Well, yes, but I think you'll have to give up the single-mailbox model and
| go with the message-per-file model.
You don't *have* to; the directory-as-folder/file-per-message model is easier
to implement this with [and the code is already in the procmailex(5) man
page], but that setup isn't always practical, especially under OSes that
allocate ridiculous minimum amounts of storage space for every file and for
every directory's catalogue.
Code to handle it with plain folders has been posted here before; I wrote
some and posted it barely a month ago.