This transport delivers into a maildir. The maildirs location may be given as
the dir argument to the constructor, and defaults to Maildir in the
current directory (at the time of transport initialization).
If the directory does not exist, it will be created.
By default, three headers will be added:
* X-Email-Sender-From - the envelope sender
* X-Email-Sender-To - the envelope recipients (one header per rcpt)
* Lines - the number of lines in the body
These can be controlled with the add_lines_header and
add_envelope_headers constructor arguments.
The Email::Sender::Success object returned on success has a filename
method that returns the filename to which the message was delivered.