Q: message editing in ANY FOLDER: which email clients allow it (Unix
platform, opensource) ?

All email clients (known to me) allow to edit messages only in ONE 
special folder, named Dratfs, or Outgoing, or like).

I am looking for a client which: ALLOWS MESSAGE
EDITING IN-PLACE IN ANY FOLDER.

Following clients (I tried them) .. they do not qualify:
- Netscape 4.7 - Netscape 6 -  Mozilla - Eudora - Pico - MailWarrior -
- Outlook - Outlook Express -- KDE kmail -- tkrat  
(ask me clarification if you think any of them do).
(And responses to use PIM which is not email client does not qualify
either.)

This questions is about Unix platform only (opensource). Client must
be GUI,
not console teletype-like.

Bonus point 1 : if client exists both for Windows and Unix platforms,
and is opensource or freeware
Bonus point 2: if client supports IMAP and supports this
feature(edit-in-place)
for IMAP folders also
Bonus point 3: if client allow CREATION of draft messages directly in
in ANY FOLDER

Additional explanation: I keep personal records inside email client.
I edit those "self-messages" often.
To drag message to Drafts, then drag message back every time I need to
edit
personal message is utterly inconvenient.
And this question has nothing to do with "forging" or faking messages.

Try this one

http://www.polarbar.org/features.html

VM, which is an Emacs package, will let you do everything you list
except the "create a message in any folder".  For that you'd have to
send yourself mail and file it in the right place.

It works on Unix (text console and X) and Windows, is Free Software,
and supports IMAP.

I don't know whether you'd consider it to have a GUI or not.  You can
do everything with a mouse using the menus and clicking on messages,
but it looks rather textish, since Emacs (both FSF Emacs and XEmacs)
are mostly text-oriented.

SquirrelMail

SquirrelMail.org

Open source, GNU license, browser based client.  Requires web server
with PHP, uses IMAP, has tons of options including global addresses
books, spam filters, more.