From MattWiki

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 and improved even further in v2.0.[1]

dbox can be used in two ways:

  1. single-dbox (sdbox in mail location): One message per file, similar to Maildir. For backwards compatibility, dbox is an alias to sdbox in mail_location.
  2. multi-dbox (mdbox in mail location): Multiple messages per file, but unlike mbox multiple files per mailbox.

One of the main reasons for dbox's high performance is that it uses Dovecot's index files as the only storage for message flags and keywords. This means that indexes don't have to be "synchronized". Dovecot trusts that they're always up-to-date (unless it sees that something is clearly broken).