Thanks to your quick reply.
I set :
domainBackupMX = yes
add the IP address of Exchange_Server to mynetworks in /etc/postfix/main.cf
I try with a exist mailbox i get this :
to=<firstname.lastname@example.org>, relay=dovecot, delay=0.09, delays=0.01/0.03/0/0.05, dsn=2.0.0, status=sent (delivered via dovecot service)
But it's don't arrive in the xxxx@Exchange_Server
If I try to send a mail to a mailbox which not apear in the ldap database I get this:
Recipient address rejected: User unknown in relay recipient table; from=<email@example.com> to=<xxxxx@exchange_server> proto=ESMTP helo=<mail-ew0-f225.google.com>
How can I get the job done ???