After an afternoon on google I've found something what should be ok:
-I'll add in master.cf:
hold unix - - n - - smtp
-I'll add in main.cf this lines:
-in main.cf, where there is:
transport_maps = proxy:mysql:/etc/postfix/mysql-virtual_transports.cf
I'll write in that database
Now all the emails that I'll try to send from the local mailhost are queued in deferred transport and I've to send them manually.
The only thing now is: how can I put all the email that I'll try to send to "hold" insted of "defer"?
I know the command to do it one by one, but I would like to have this configuration directly by postfix...