Maildir is a directories structure to for storing message. It is an alternative for old storage format: mbox. Maildirs were originally implemented in the Qmail mail server, supposedly to address the inadequacies of mbox files. Instead storing it to one single huge file like mbox method, each email will be placed in one file. By storing it in multiple file,multiple processes can use maildir at the same time.
Maildir directories usually has three sub-directories named
cur. When MDA stores an email, it place the email on
tmp directory then moving it to
cur directory was place for email that have been seen by the user's mail-reading program.