Pour info, j'ai trouvé un moyen.
Voici ma nouvelle config.
smtpd_recipient_restrictions = reject_non_fqdn_recipient,
reject_unknown_sender_domain,
reject_non_fqdn_sender,
reject_unknown_recipient_domain,
reject_invalid_helo_hostname,
reject_unlisted_recipient,
reject_unlisted_sender,
permit_mynetworks,
permit_sasl_authenticated,
reject_non_fqdn_helo_hostname,
reject_unauth_destination,
check_sender_access hash:/etc/postfix/access,
reject_rbl_client zen.spamhaus.org,
reject_rbl_client bl.spamcop.net,
permit
dans la table check_sender_access j'ai ajouté le serveur de mail de ubuntu.