Quoting W B Hacker:
> The 'classical' has them as subdirs under .INBOX, and/or further subdirs under
> subdirs etc. ad (some finite, but arbitrarily large, OS & fs-sepcific) limit.
http://www.inter7.com/courierimap/README.maildirquota.html:
--- cut --
Can folders have subfolders, defined in a recursive fashion? The answer
is no. If you want to have a client with a hierarchy of folders, emulate
it. Pick a hierarchy separator character, say ":". Then, folder foo/bar
is subdirectory .foo:bar.
--- cut --
The commonly used separator is "."
