[Dovecot] Mailbox formats

[Kenneth Porter]

I'm currently using mbox and mbx with UW-IMAP, mbox for space efficiency 
and mbx for folders with lots of messages for time efficiency without a 
large space cost. I've avoided maildir because it has a relatively high 
space cost; mbox has no inter-message cost while maildir wastes on average 
half a sector plus an inode for each message.

What other formats are there, and has anyone come up with a good format 
that gives both good time and space characteristics without sacrificing 
reliability?

(I'm aware of the file format list in the UW-IMAP distro: 
<http://www.washington.edu/imap/documentation/formats.txt.html>)