Maildir

Maildir is a format for storing e-mail messages that, in contrast to the mbox format, stores each message in a separate file, making it useful for multi-user systems with file locking, in which storing all messages in a single file would only allow one user to change it at a time, while a multi-file system could be used by different users simultaneously as long as they were accessing different messages. A structure of subdirectories corresponds to the mailbox folder structure in which the messages are stored.

The Maildir format is documented in an open spec so it can be used by multiple mail client and server software on different platforms, though there are platform compatibility issues stemming from the fact that the format uses colons as part of filenames, and this is not allowed in all platforms (MS Windows disallows it, for instance). Alternative delimiters such as dash (-) or semicolon are used when necessary, creating variant and nonstandard versions of the format.

Links

 * Wikipedia article
 * Maildir spec
 * Maildir man page