By default, the manager's open() method will look for the
folder whose name is found in the environment variable MAIL.
The names may also be supplied as first argument or via the folder
option.
Folder names may start an '=', which will be replaced by
the default folder location (folder type specific, overruled by the value
of folderdir). Names may also be shaped as url, which is
especially useful when you wish to supply complex folder information
(for instance required for a POP3 connection) via the MAIL
environment variabele.