This documentation is for Dovecot v2.x, see wiki1 for v1.x documentation.

Mbox Mailbox Format

Usually UNIX systems are configured by default to deliver mails to /var/mail/username or /var/spool/mail/username mboxes. In IMAP world these files are called INBOX mailboxes. IMAP protocol supports multiple mailboxes however, so there needs to be a place for them as well. Typically they're stored in ~/mail/ or ~/Mail/ directories.

The mbox file contains all the messages of a single mailbox. Because of this, the mbox format is typically thought of as a slow format. However with Dovecot's indexing this isn't true. Only expunging messages from the beginning of a large mbox file is slow with Dovecot, most other operations should be fast.


Locking is a mess with mboxes. There are multiple different ways to lock a mbox, and software often uses incompatible locking. There are at least four different ways to lock a mbox:


Another problem with dotlocks is that if the mailboxes exist in /var/mail/, the user may not have write access to the directory, so the dotlock file can't be created. There are a couple of ways to work around this:


If multiple lock methods are used, which is usually the case since dotlocks aren't typically used for read locking, the order in which the locking is done is important. Consider if two programs were running at the same time, both use dotlock and fcntl locking but in different order:

Now both of them are waiting for each others locks. Finally after a couple of minutes they time out and fail the operation.

Locking used by different software

Dovecot's Metadata

Dovecot uses C-client (ie. UW-IMAP, Pine) compatible headers in mbox messages to store metadata. These headers are: