Maildirstyle

Maildirstyle refers to an approach for storing email messages in a filesystem hierarchy based on the Maildir format. It is used by several mail servers and clients as an alternative to the traditional mbox format, prioritizing per-message file storage and straightforward concurrent access.

In the Maildir style, each mailbox is represented by a directory named Maildir containing three subdirectories: cur, new, and tmp. The new directory holds messages that have recently arrived but have not yet been seen by clients; the cur directory holds messages that have been delivered and may have status flags; the tmp directory is used during delivery to ensure atomic writes. This organization avoids the single-file bottleneck associated with mbox and allows multiple processes to access different messages simultaneously.

Messages are stored as individual files within cur or new. Each filename encodes a unique delivery identifier, and after a colon, there is a two-digit entry followed by a comma and a string of status flags. The exact meaning of the flags varies by implementation, but they generally convey states such as unread/read, answered, flagged, or draft. Because messages are separate files, applications can update flags by renaming or modifying these per-message files without locking an entire mailbox.

Advantages of the Maildir style include robustness to corruption, simpler concurrent access, and ease of incremental backups or transfers. It generally performs well for large mailboxes and high delivery rates. Limitations can include the need for compatible indexing or search tools, and some legacy clients or servers may require adapters to work with Maildir structures. Maildirstyle was popularized by qmail’s Maildir and has since been widely adopted by modern mail systems such as Dovecot and Courier.