A folder to access MH format mail (1 file is 1 mail).
Normally, directory-name is an relative path to the variable
elmo-localdir-folder-path (default is `~/Mail'), but if it
starts with `/' or `~', then it is treated as an absolute path
(this is also true for drive-letters).
Message number is used for the name of the message file.
+inbox -> `~/Mail/inbox' +from/teranisi -> `~/Mail/from/teranisi' +~/test -> `~/test'
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