	     HOWTO get balsa managing existing MH folders

Author: Stephan C . Buchert <scb@stelab.nagoya-u.ac.jp>

Many users of balsa have an archive of their previously received,
drafted, sent, deleted ... mails. Of course they want to manage these
with balsa. Depending on the mail system that has been used the
already existing messages are stored in different ways:

1) in the local computer as so-called mbox files
2) in the local computer as so-called MH folders
3) in a server as so-called IMAP folders

This list is perhaps not complete.

Here I describe how I got balsa managing my emails that are stored in
MH folders, nr 2 above.

The MH (Message Handling) system stores each e-mail message in a
separate file. A directory with one or several files containing MH
emails is an MH folder. Thus a part of the UNIX file system itself
represents the structure of an MH mail archive. The root of this
structure is typically at "/home/user/Mail", where "user" stands for
the login name. In contrast to this, mbox files (nr 1 above) contain
typically more than one message. An mbox file with only one message is
not an MH message, and vice versa.

MH mail data can be managed using the "low level" nmh tools, which are
included in most Linux distributions, by exmh, which provides a GUI to
the nmh tools, by two different packages for Emacs, namely mh-mail and
Mew, and by others.

Also Balsa can be used to manage your MH folders, as well as mbox
files and IMAP folders. The latter two are mailboxes in balsa, but an
error results if a balsa mailbox path is configured to an MH mail
folder. Rather, my "Local Mail" in the window for "Mail Servers" from
"Preferences" says "/home/scb/Mail" and balsa shows nicely all my MH
folders which are under "/home/scb/Mail".

There was another minor hurdle, before balsa really displayed my MH
mails:

Obviously balsa identifies MH mail folders by the existence of a file
".mh_sequences" in these folders. If, for example, the Emacs mh-mail
package has been used before balsa, then there is no problem. However,
I use a system called MEW (https://www.mew.org) which lets me see mail
with Japanese characters using Emacs/Mule. Mew doesn't create files
".mh_sequences" (but rather files ".mew-cache"). Therefore I had to
create (touch) files ".mh_sequences" in each MH folder. Concerning
balsa these files ".mh_sequences" can be empty, they only must exist.

Once balsa recognizes the MH folders as described above, it can handle
both MH folders and mailboxes, transfer messages between these two etc.
