First of all thanks for the reply,
the problem is that zarafa doesn't store e-mails in MailDir format but in a MySQL database so transferring the folders doesn't help me.
Eventually I solved this way:
- Access the old server via IMAP in Outlook.
- Export messages to the ".pst" file.
- Access the new server via IMAP in Outlook.
- Import emails from the ".pst" file.
I hope this can be useful for others with my problem;)
Greetings,
Riccardo.