a mail storage format in which every message is kept in a separate file, and meta-information flags (like which messages have been seen by the user) is manipulated by renaming the files, instead of altering the contents. This format is safer than mbox when multiple programs are accessing the mail store at the same time, and faster tham mbox when messages are deleted or flags are changed. Maildir format is supported by many mailreaders and POP/IMAP servers.