dbox is Dovecot's own high-performance mailbox format. The original version was introduced in v1.0 alpha4, but since then it has been completely redesigned in v1.1 series.
dbox can be used in two ways:
One message per file, similar to Maildir.
Multiple messages per file, but unlike mbox multiple files per mailbox. (This isn't currently fully working.)
One of the main reasons for dbox's high performance is that it uses Dovecot's index files as the primary storage for message flags and keywords. They're still backed up to the dbox files once in a while though. Using index files as the primary storage also means that the indexes don't have to be "synchronized". Dovecot trusts that they're always up-to-date (unless it sees that something is clearly broken).
Unlike Maildir the message file names don't change. This makes it possible to support storing files in multiple directories or mount points. dbox supports looking up files from "altpath" if they're not found from the primary path. This means that it's possible to move older mails that are rarely accessed to cheaper (slower) storage.
dbox supports a quick migration from Maildir format. It basically works by renaming Maildir/cur/ directory as the dbox's mail directory and then moving files from Maildir/new/ directory and Dovecot's control and index files there as well. The maildir files are sometimes later then converted to actual dbox files when their message flag changes are decided to be backed up (if there are no changes, they're never converted currently).
dbox storage is extensible, so in future there will be other extensions. Some things that are planned:
- Single instance attachment storage. If multiple mailboxes/users have the same attachment, it's stored only once in disk.
- Multiple mailboxes in one dbox storage. This makes copying messages from one mailbox to another a simple metadata update.
Multiple messages per file mode isn't currently fully functional, so it can't be enabled without modifying sources.