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 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.
Read more at: https://en.wikipedia.org/wiki/Maildir