[Dovecot] mailbox format w/ separate headers/data

[Frank Cusack]

In the future, it would be cool if there were a mailbox format (dbox2?)
where mail headers and each mime part were stored in separate files.
This would enable the zfs dedup feature to be used to maximum benefit.

In the zfs filesystem, there is a dedup feature which stores only 1 copy
of duplicate blocks.  In a normal mail file, the headers will be
different for each recipient and the chances of the content of the message
being able to be dedup'd are close to zero, because the differences in
header length changes the block boundaries for the rest of the message.
But if each mime part is stored in a separate file, you get massive
compression "for free".

-frank