I've just migrated to a Postfix system and can't seem to stop the backscatter messages to unknown addresses on the site.

I have a file, validrcpt, that lists all the valid emails on the site - about eight of them. Yet when a message is sent to a non-existent address, instead of just dropping it, postfix is replying with a "Recipient address rejected: User unknown in virtual mailbox table" email.

Do I have something set wrong? I've read http://www.postfix.org/BACKSCATTER_README.html but unless I'm caffeine deficient, I don't see what's happening and perhaps I'm just to used to my old qmail setup. Here's postconf -n:

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
append_dot_mydomain = no
biff = no
broken_sasl_auth_clients = yes
config_directory = /etc/postfix
content_filter = smtp-amavis:[127.0.0.1]:10024
home_mailbox = Maildir/
inet_interfaces = all
inet_protocols = ipv4
local_recipient_maps = hash:/etc/postfix/validrcpt
mailbox_command = /usr/lib/dovecot/deliver -c /etc/dovecot/dovecot.conf -m "${EXTENSION}"
mailbox_size_limit = 0
mydestination = localhost
myhostname = localhost
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
myorigin = /etc/mailname
policy-spf_time_limit = 3600s
readme_directory = no
recipient_bcc_maps = hash:/etc/postfix/recipient_bcc
recipient_delimiter = +
relay_recipient_maps = hash:/etc/postfix/relay_recipients
relayhost =
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtp_use_tls = yes
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
smtpd_recipient_restrictions = permit_mynetworks,permit_sasl_authenticated,reject_unauth_destination,check_policy_service unix:private/policy-spf,reject_rbl_client zen.spamhaus.org,reject_rbl_client bl.spamcop.net,reject_rbl_client cbl.abuseat.org,check_policy_service inet:127.0.0.1:10023
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
smtpd_sasl_auth_enable = yes
smtpd_sasl_authenticated_header = yes
smtpd_sasl_local_domain = $myhostname
smtpd_sasl_path = private/dovecot-auth
smtpd_sasl_security_options = noanonymous
smtpd_sasl_type = dovecot
smtpd_sender_restrictions = reject_unknown_sender_domain
smtpd_tls_auth_only = yes
smtpd_tls_cert_file = /etc/dovecot/dovecot.pem
smtpd_tls_key_file = /etc/dovecot/private/dovecot.pem
smtpd_tls_mandatory_ciphers = medium
smtpd_tls_mandatory_protocols = SSLv3, TLSv1
smtpd_tls_received_header = yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes
tls_random_source = dev:/dev/urandom
virtual_gid_maps = static:5000
virtual_mailbox_base = /home/vmail
virtual_mailbox_domains = digitalhit.com
virtual_mailbox_maps = hash:/etc/postfix/vmaps
virtual_minimum_uid = 1000
virtual_uid_maps = static:5000