[Dovecot] Separate access to different "folders" of the same mailbox?

[Alexander Chekalin]

Hello Timo and everyone,

I think on such an idea that may save me a bit of space and time:

in my company we have a mailbox that holds a copy of every message that 
our SMTP processed. While it eats a lot of space, it saved us several 
times when, you may imaging, user "suddenly" deleted the most important 
message in his life and call our IT guys for help. The mailbox contains 
"folders" for each day (like 2011-02-10), which keeps mailings for that 
day only.

Even with this, it is sometime hard to work with that "keep all" box 
(basically, it is slow as the size is huge). What I'd like to add is 
some kind of "virtual folders" (for each of our internal users) that 
list messages from and to his address. And I'd like to permit every user 
to see (that is, read only mode) only "his" such a virtual folder in our 
"keep all" box, so user can see his old messages on his own.

Looks like
1) I need an ACL to limit user access to "their" folders and somehow 
hide other folders at all
2) Have such virtual folders work with huge ammount of mailings (kind of 
300 Gb), separated in maybe 1000 folders (per days). I really afraid it 
will be slo-o-o-ow search, even with indexes (I mean, virtual folder 
that is defined as search over 1000 folders will be not that fast).

The mailbox is maildir by nature. How can I archive these goals with 
smallest load possible, and if it is possible at all?

Thank you,
   Alexander Chekalin