You seem to have a problem with this line:
smtpd_client_restrictions = reject_unknown_client
Uncomment this.
Here's my resctrictions on my main.cf file:
# rules restrictions
smtpd_client_restrictions =
smtpd_helo_restrictions =
smtpd_sender_restrictions =
permit_mynetworks,
permit_sasl_authenticated,
reject_non_fqdn_sender,
reject_unknown_sender_domain,
check_sender_access hash:/etc/postfix/sender_access
smtpd_recipient_restrictions = check_policy_service inet:127.0.0.1:10031,
permit_sasl_authenticated,
permit_mynetworks,
reject_unauth_destination,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_unknown_recipient_domain,
check_policy_service unix:private/policyd-spf
#reject_rhsbl_helo dbl.spamhaus.org,
#reject_rhsbl_reverse_client dbl.spamhaus.org,
#reject_rhsbl_sender dbl.spamhaus.org,
#reject_rbl_client zen.spamhaus.org
# uncomment for realtime black list checks
# ,reject_rbl_client zen.spamhaus.org
# ,reject_rbl_client bl.spamcop.net
# ,reject_rbl_client dnsbl.sorbs.net