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 following filesystem folder structure:
mail \ - cur - new - tmp - .INBOX \ - cur - new - tmp
(and likewise for .Sent and .Trash)
Furthermore, assuming we have a deeper folder structure, say, a top-level email folder "Chronological/2012/09", it would be stored as follows:
mail \ - cur - new - tmp - .Chronological.2012.09 \ - cur - new - tmp
Is this correct? Some docs are a bit ambiguous about this, and I've seen implementations vary: some store the top-level folders without the leading dot, and some create a new filesystem subdirectory for each subfolder level (rather than using the dots to denote a new logical level).