I upgrated from kde 4.6.3 to 4.6.4 that now have the 4.6.0 version of kontact and kmail.
The akonadi message say the migration was success.
But I can't access kmail that crash at start.
How can I access with a fresh new configuration? Can I recover the old mails and settings?

Remove (or rename) kmail2rc from the KDE config directory (usualle $HOME/.kde/share/config or $HOME/.kde4/share/config).

KMail should the startup empty.

This procedure doesn't solve the problem, same message

L'esecuzione di KMail verrà terminata a causa di un problema.
L'errore è:
Recupero della collezione di risorse non riuscito.

Can you please provide a translation of that message?

Translation by Google Translation:

Running KMail will be terminated because of a problem.
The error is:
Recovery of the collection of resources failed.

Try the solution at the end of the bug report. Worked for me. https://bugs.kde.org/show_bug.cgi?id=259355

I configured it to use my gmail account. It fetched a lot of mails, but then stopped. And now it's not receiving any more mails. The latest mail in the inbox is from march 2011.

Any ideas? I didn't have any previous configurations as I deleted them all according to the solution in the bug report.

Have you checked viewtopic.php?f=20&t=95520#p198122 ?