maildir

Maildir is a mailbox format used for storing email messages as individual files within a directory tree. It was introduced for the qmail mail transfer agent and designed to provide safe, concurrent access to messages. A standard Maildir mailbox contains three subdirectories: new, cur, and tmp. Each message is stored as a separate file, with the file name chosen to be globally unique, typically incorporating a timestamp, host name, and process identifier. The new subdirectory holds messages that have just been delivered and not yet seen by a client; the cur subdirectory holds messages that have been delivered and may have status flags; the tmp subdirectory is used during the delivery process for temporary storage until the message is finalized.

Delivery and status handling: when a message is delivered, the MTA writes a temporary file under tmp and then atomically moves it into either new or cur. Clients or servers can mark messages as seen or otherwise modify their status by renaming the file within cur to include a status suffix, such as :2,RS, where the letters denote flags like seen or replied. The exact flag semantics can vary by implementation, but the general approach enables safe concurrent access because files are manipulated atomically on most filesystems.

Adoption and variants: Maildir is widely supported by many MTAs and mail servers, and it remains a common choice for local delivery. A related extension, Maildir++, broadens support for subdirectories and additional features in some implementations.

See also: Mbox, Maildir++.