prova con le restrizioni applicate su questo file a sender, recipient e client:

smtpd_banner = $myhostname ESMTP $mail_name
smtp_helo_required = yes
disable_vrfy_command = yes
content_filter = vscan:[127.0.0.1]:10024
biff = no
append_dot_mydomain = no
myhostname = linuxbox.xxx.net
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
canonical_maps = hash:/etc/postfix/canonical
sender_canonical_maps=hash:/etc/postfix/sender_canonical
myorigin = /etc/mailname
mydestination = smtp.xxx.net, linuxbox.xxx.net, localhost.xxx.net, localhost, xxx.net
mynetworks = 127.0.0.0/8 192.168.1.0/24
mailbox_command = /usr/bin/spamc | procmail -a "$EXTENSION"
mailbox_size_limit = 51200000
recipient_delimiter = +
inet_interfaces = all

smtpd_recipient_restrictions =
permit_sasl_authenticated,
reject_invalid_hostname,
reject_non_fqdn_hostname,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_unknown_sender_domain,
reject_unknown_recipient_domain,
reject_unauth_pipelining,
permit_mynetworks,
reject_unauth_destination,
reject_rbl_client zombie.dnsbl.sorbs.net,
reject_rbl_client relays.ordb.org,
reject_rbl_client opm.blitzed.org,
reject_rbl_client list.dsbl.org,
reject_rbl_client sbl.spamhaus.org,
reject_rbl_client blackholes.easynet.nl,
permit

smtpd_sender_restrinctions =
warn_if_reject,
check_sender_access,
permit_sasl_authenticated,
permit_mynetworks,
reject_unauth_destination

stmpd_client_restrictions =
permit_mynetworks

mime_header_checks=pcre:/etc/postfix/mime_header_checks
header_checks = regexp:/etc/postfix/header_checks



p.s: xxx è il nome della ditta