[CentOS] Postfix - message queue filling with Host or name not found - try again

Hi list,
I have noted over the last week or so my DNS servers are dumping lots of 
messages for bogus domain lookups. Examining the postfix queue with 
postqueue -p:  I see many
(Host or domain name not found. Name service error for 
name=bdgiedjhea.po6e4ina.com type=MX: Host not found, try again)
                                         Jake at bdgiedjhea.po6e4ina.com
My question - why does this stay in the message queue - why not dumped 
back with message undeliverable or dropped?
I understand this is probably related to my config - which follows:
<main.cf>
queue_directory = /var/spool/postfix
command_directory = /usr/sbin
daemon_directory = /usr/libexec/postfix
mail_owner = postfix
myhostname = <mumble>
mydomain = <mumble>
myorigin = $mydomain
inet_interfaces = all
inet_protocols = ipv4
proxy_interfaces = <mumble>
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain,
        mail.$mydomain, www.$mydomain, ftp.$mydomain
unknown_local_recipient_reject_code = 550
mynetworks = 192.168.230.0/24, 127.0.0.0/8
relay_domains =
virtual_alias_domains = hash:/etc/postfix/virtual_alias_domains
virtual_alias_maps = hash:/etc/postfix/virtual_alias_maps
smtpd_helo_required = yes
smtpd_delay_reject = yes
strict_rfc821_envelopes = yes
smtpd_helo_restrictions =
   permit_mynetworks,
   reject_non_fqdn_helo_hostname,
   reject_invalid_helo_hostname,
   permit
smtpd_sender_login_maps = hash:/etc/postfix/smtpd_sender_login_map
smtpd_client_restrictions =
  check_client_access hash:/etc/postfix/access
smtpd_recipient_restrictions =
  reject_unauth_pipelining,
  reject_non_fqdn_recipient,
  reject_non_fqdn_sender,
  reject_unknown_sender_domain,
  reject_unknown_recipient_domain,
  permit_mynetworks,
  permit_sasl_authenticated,
  reject_unauth_destination,
  check_recipient_access hash:/etc/postfix/roleaccount_exceptions,
  reject_invalid_hostname,
  check_helo_access pcre:/etc/postfix/helo_checks,
  reject_rbl_client sbl-xbl.spamhaus.org,
  permit
smtpd_sender_restrictions =
  permit_mynetworks,
  permit_sasl_authenticated,
  reject_non_fqdn_sender
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
tls_random_exchange_name = /var/spool/postfix/prng_exch
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_use_tls = yes
smtpd_tls_security_level = may
smtpd_tls_auth_only = yes
smtpd_tls_key_file = /etc/pki/tls/private/ssl.key.private.decrypted
smtpd_tls_cert_file = /etc/pki/tls/certs/<mumble>
smtpd_tls_CAfile = /etc/pki/tls/certs/sub.class2.server.ca.pem
smptd_tls_loglevel = 2
smtpd_tls_received_header = yes
smtpd_sasl_tls_security_options = noanonymous
smtpd_sasl_security_options = noanonymous
mailbox_size_limit = 102400000
message_size_limit = 40960000
in_flow_delay = 1s
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
home_mailbox = Maildir/
content_filter=amavisfeed:[127.0.0.1]:10024
debug_peer_level = 2
debugger_command =
         PATH=/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin
         xxgdb $daemon_directory/$process_name $process_id & sleep 5
sendmail_path = /usr/sbin/sendmail.postfix
newaliases_path = /usr/bin/newaliases.postfix
mailq_path = /usr/bin/mailq.postfix
setgid_group = postdrop
html_directory = no
manpage_directory = /usr/share/man
sample_directory = /usr/share/doc/postfix-2.2.2/samples
readme_directory = /usr/share/doc/postfix-2.2.2/README_FILES
</main.cf>


Any suggestions appreciated.
TIA rob
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