Postfix configuration error (451 4.3.5 Server configuration problem)

Hi! Lately, I've been seeing these errors in my log. Some people have reported that mails are being returned undelivered.

Jan 21 00:02:45 localhost postfix/smtpd[28622]: NOQUEUE: reject: RCPT from mail.sunbeam.co.nz[203.167.253.34]: 451 4.3.5 Server configuration problem; from=<****@aol.com> to=<****@****.ie> proto=ESMTP helo=<quantum.sunbeam.co.nz>

I know this error is very generic, but I couldn't find anything else in my syslog, and I was hoping a Postfix expert or experts could help troubleshoot.

Any research I've done into this error seems to be pointing towards Postfix Policy Daemon - postfix-policyd-spf-perl - for SPF checking. I followed this guide to implement this and up until now it seems to have been working:

http://www.howtoforge.com/postfix_spf

Thanks in advance for any help. My Postfix main.cf is attached below.

# see /usr/share/postfix/main.cf.dist for a commented, fuller
# version of this file.
 
# Do not change these directory settings - they are critical to Postfix
# operation.
command_directory = /usr/sbin
daemon_directory = /usr/lib/postfix
program_directory = /usr/lib/postfix
 
home_mailbox = Maildir/
 
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
setgid_group = postdrop
biff = no
 
smtpd_helo_required = yes
 
# appending .domain is the MUA's job.
append_dot_mydomain = no
myhostname = mail.localhost.net
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
relayhost =
mynetworks = 127.0.0.0/8
mailbox_command = procmail -a "$EXTENSION"
recipient_delimiter = +
mydestination = $myhostname
relay_domains = $mydestination
 
smtpd_client_restrictions =
        check_client_access hash:/etc/postfix/access
 
smtpd_sender_restrictions =
        hash:/etc/postfix/access, reject_unknown_sender_domain
 
smtpd_recipient_restrictions = hash:/etc/postfix/access, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, check_policy_service unix:private/policy, reject_rbl_client bl.spamcop.net
 
default_privs = mail
#mailbox_size_limit = 0
message_size_limit = 51200000
mailbox_size_limit = 102400000
smtpd_error_sleep_time = 0
 
#20060923
header_checks = regexp:/etc/postfix/header_checks
 
### CONFIXX POSTFIX ENTRY ###
virtual_maps = hash:/etc/postfix/confixx_virtualUsers, hash:/etc/postfix/confixx_localDomains
### /CONFIXX POSTFIX ENTRY ###
 
smtpd_sasl_local_domain = $myhostname
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
virtual_alias_maps = hash:/etc/postfix/confixx_virtualUsers, hash:/etc/postfix/confixx_localDomains
inet_protocols = ipv4
#smtpd_sasl_path = /etc/postfix/sasl

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