Maildir format

Each mail message is encoded as:

• a header section,
• a blank line,
• the body section

However unlike mbox each message has its own file in a directory.

• mbox files

A whole mailbox has three directories within itself: new, cur, and tmp.

tmp: when storing new messages they are written here, flushed, then atomically moved to 'new.'

new: messages deposited to a mailbox but not acknowledged by the mail client belong here.

cur: messages that mail software has acknowledged as newly arrived are moved to this directory. mail lives here until the user moves or deletes it.

Atomic moves are typically done with `rename` or by hardlinking then unlinking the original.