(GUI) Mail Client that can edit (or view) multiple original messages simultaneously

I'm looking for a (GUI) email client which:

* allows me to edit original messages
* stores them in mbox format
* allows me to "open" multiple messages at the same time, for
viewing or editing

I know this is probably a perversion of normal email usage--it's
actually for an application for which I'd like to use mbox files as a
"quick and dirty" alternative to a real database.

(I also recognize that I could use maildir files to do what I
want--there are a few reasons that I'd prefer to use mbox files (among
them: multiple records in a single file which I can load into nedit as
an alternative means of processing and scroll very quickly between
them, and use other tools that I've built using nedit macros).)

I've found some approaches that almost do what I want, for example, I
can use mail/nail, set my EDITOR to nedit and set the writebackedited
variable. The problem is that I can open only one email at a time
within a single instance of mail/nail for editing (or viewing), and if
I open multiple instances of mail/nail, I'm in that situation of "last
(editor) to save, wins" (i.e., only one of the edited records is
properly saved, the last one that I close).

(For the record, the way that I do that is first set the following in
/etc/nail.rc:

set writebackedited
set EDITOR="/usr/bin/nedit"

and then issue commands like the following:

[rhk@s14 ~]$ echo "e 6" | mail -f /rhk/Mail/sample.email &
[rhk@s14 ~]$ echo "e 7" | mail -f /rhk/Mail/sample.email &
)

Randy Kramer

I am, of course, also considering making the "big switch" to a real
relational database (I'm playing with MySQL, and I've found that it
does have some FULLTEXT indexing and search capabilities, but I supect
these are not quite what I've grown to expect from recoll (and recoll
won't currently work for a MySQL database, but will work for mbox (and
maildir, iiuc) files).