[Dovecot] New mailbox format

[dean gaudet]

oh also...  somethign else i've thought about a lot in relation to the 
"archive format" i described are "virtual" folders:  a virtual folder is a 
subset of the index which is presented via IMAP as a "real" folder.  
(there was a DEC research mail system which did this ages ago, i don't 
remember its name now.)

there are many uses for this:

- a tool could perform a sophisticated search that's not supported by IMAP 
  and present it as a virtual folder so that any IMAP MUA can see the 
  results.

- for an archive containing 100Ks of messages you could create date
  subrange views again so that any IMAP MUA can read it without
  performance difficulties.  consider the application of this to
  public mailing list archives:  an archive mailbox presented via anon
  read-only IMAP so that you can browse it directly with your favourite
  MUA (inluding webmails which talk IMAP for a google indexable web
  interface).

- there's technically only the need for a single index and collection of
  mailboxes (for each user) -- delivery filtering tools (i.e.
  procmail/maildrop) could place the message into one of several virtual
  folders which the user treats as inboxes.

-dean