Hey Nobumi,

On Jun 20, 2013, at 20:52, Nobumi Iyanaga <[log in to unmask]> wrote:
> I use Mail.app since OS X began, but it seems to have some grave problems: the Mail folder, in my Library, is becoming huge -- it is now 20.3 GB (20,321,432,464 bytes) with 1,056,521 items in it.

Yeowch.  My mail folder as of today:  1,243,049,958 bytes (1.39 GB on disk) for 99,811 items

> I would like to be able to clean up this mail folder, in a safe way (to not corrupt the mail database...). I found a script set called MailScripts (2.10.3), which seems to contain an AppleScript application, and has a command named "Remove Duplicates", but the web page says that some of the scripts do not work on Lion and later (I am using the newest OS)...

I would be very reluctant to use Applescript for this job.  Even if it works it's likely to take many hours.

Whatever you do make sure to have a full backup before you start.

I've been looking at this for some time: http://www.mailsteward.com

It is supposed to be able to remove duplicates and export back out to an mbox file (which can import back into Mail).

Here are a couple of other resources:

http://c-command.com/spamsieve/manual-ah/how-can-i-rebuild-apple

http://www.macobserver.com/tmo/answers/how_to_clean_up_and_rebuild_apple_mail_data

--
Best Regards,
Chris