Maildir

The Maildir e-mail format is a common way of storing e-mail messages, where each message is kept in a separate file with a unique name, and each folder is a directory. The local filesystem handles file locking as messages are added, moved and deleted. A major design goal of Maildir is to eliminate program code having to handle locking, which is often difficult.

Specifications

A Maildir directory (often named Maildir) usually has three subdirectories named tmp, new, and cur.

Maildir

The original Maildir specification was written by Daniel J. Bernstein, the author of qmail, djbdns, and other software. Although the original specification was written specifically for Bernstein's qmail, it is general enough to be implemented in many programs.

Maildir++

Sam Varshavchik, the author of the Courier Mail Server and other software, wrote an extension to the Maildir format called Maildir++ to support subfolders and mail quotas. Maildir++ directories contain subdirectories with names that start with a '.' (dot) that are also Maildir++ folders. This extension is therefore a violation of the Maildir specification, which provides an exhaustive list of the possible contents of a Maildir, however it is a compatible violation and other Maildir software supports Maildir++.