Maildir is a format for an e-mail spool that does not require file locking to maintain message integrity, because the messages are kept in separate files. It consists of a directory named Maildir with three subdirectories, tmp, new, and cur.
The process that delivers mail writes it to tmp with a filename that consists of the concatenation of the process ID, the time, and the hostname, then when it is finished moves it to new. The process that reads mail moves messages from new to cur before reading them, appending a comma and flags to the filename.
Software that supports Maildir includes: