I have recently installed the FC 17, running Kmail 4.8.4.

I have certainly messed up things while importing my Kmail folders from a previous installation, and now inbox is not working. It shows that there are more than 30000 messages (!), but when I try to click the directory I get the message "unable to fetch item from backend".

Thus, I would like to reinstall Kmail, and import the previous folders in the correct way, i.e. importing compressed folders. But I do not know how to uninstall and reinstall Kmail, to start a fresh new installation and I have not found any documentation.

Any suggestions?

you wouldn't reinstall it, that wouldn't help you at all. the installation will be just fine, its your own use configuration of it that's broked.

It looks like your config file is going to be at ~/.kde/share/config/kmailrc and any imported mail would be in ~/.Mail by default I think.

I agree: I would prefer simply to re-configure....

But I do not have any ~/.Mail directory: everything seems to be under ~/.kde/share/apps/kmail/mail:
..................
drwx------. 5 antonucc antonucc 4096 Jun 28 18:49 inbox
drwx------. 6 antonucc antonucc 4096 Oct 13 2011 .inbox.directory
-rw-------. 1 antonucc antonucc 5807445 Jun 28 18:49 .inbox.index
-rw-------. 1 antonucc antonucc 46325 Jun 28 18:49 .inbox.index.ids
drwx------. 5 antonucc antonucc 4096 Feb 5 2011 infn_grid
-rw-------. 1 antonucc antonucc 3173 Feb 5 2011 .infn_grid.index
-rw-------. 1 antonucc antonucc 57 Jun 28 18:49 .infn_grid.index.ids
drwx------. 5 antonucc antonucc 4096 Feb 5 2011 IT providers
-rw-------. 1 antonucc antonucc 1521 Feb 5 2011 .IT providers.index
-rw-------. 1 antonucc antonucc 45 Jun 28 18:49 .IT providers.index.ids
..................

The problem is that if I move this direcyory away:

mv ~/.kde/share/apps/kmail/mail ~/old-kmail-mail

and change the line "first-start=true" to false in .kmailrc, and re-launch kmail, I still see the old directory structure...

I would nuke the kmailrc file too personally. But I avoid both local email clients and anything with a "k" at the start like the plague, so the two together I'm proud to be highly ignorant about!