Hi Victor, I agree that the cron job solution is not the best one... but at the moment it is the only one that I'm able to offer... (Even if with a cron job every 5 seconds(for example), I'll not lose any emails...)
As Noel Jones said before: "The documented way to tell postfix to accept mail for a domain is to put the domain in one of {mydestination, relay_domains, virtual_alias_domains, virtual_mailbox_domains}. See below for some documentation links. I don't see transport_maps listed there." So, with my configuration, if you can suggest me a possible and better solution I'll really appreciate it. Thanks a lot Michele