Maildir - Wikipedia

The Maildir e-mail format is a common way of storing email messages on a file system, rather than in a database. Each message is assigned a file with a unique name, and each mail folder is a file system directory containing these files. Maildir was designed by Daniel J. Bernstein circa 1995, with a major goal of eliminating the need for program code to handle file locking and unlocking through use of the local filesystem.[1] Maildir design reflects the fact that the only operations valid for an email message is that it be created, deleted or have its status changed in some way.

Sam Varshavchik, the author of the Courier Mail Server and other software, defined the Maildir++ extension[3][4] to the Maildir format to support subfolders and mail quotas. Maildir++ directories contain subdirectories with names that start with a '.' (dot) which are also Maildir++ folders. The extension complies with the original Maildir specification, which allows for subdirectories in addition to tmp, new and cur.

A mail delivery agent is a program that delivers an email message into a Maildir. The mail delivery agent creates a new file with a unique filename in the tmp directory.[5][6][3] At the time of its invention guaranteeing unique filenames efficiently was difficult. The original qmail[1] algorithm for unique names was:

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