what is the correct Courier / Maildir++ email (sub)folder format?

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http://unix.stackexchange.com – For representing the logical email folder structure, a "Maildir++" format exists that was first implemented by the Courier mail server, thus sometimes also being referred to as the "Courier" format. Unfortunately, some ambiguity exists in documentation and various implementations, regarding how the logical email folders are to be stored inside the main "Maildir" directory, ie. inside the main email directory containing the three subdirectories "cur", "new" and "tmp". Assuming there are three email folders, "INBOX", "Sent" and "Trash", I have understood the canonical spec to mean the followin (HowTos)