554 5.6.0 NUL Character - Way to whitelist domain?

After enabling my RBL lists a few days ago, I have had a few clients tell me that they are getting their messages rejected from my server. Today, I got a new one. I have no idea what NUL characters are and am unsure if they are normal or not. They get the following:

host /var/imap/socket/lmtp[/var/imap/socket/lmtp] said 554 5.6.0 Message contains NUL characters (in reply to end of DATA command)

Is this on their end or mine? Also, 95% of our email traffic is with about 30 domains. Is there a way to add them to a whitelist so that everything comes through no matter what? Below is my postconf -n

xserve0:~ root# postconf -n
alias_maps = hash:/etc/aliases,hash:/var/mailman/data/aliases
command_directory = /usr/sbin
config_directory = /etc/postfix
content_filter = smtp-amavis:[127.0.0.1]:10024
daemon_directory = /usr/libexec/postfix
debug peerlevel = 2
enable serveroptions = yes
html_directory = no
inet_interfaces = all
mail_owner = postfix
mailbox sizelimit = 0
mailbox_transport = cyrus
mailq_path = /usr/bin/mailq
manpage_directory = /usr/share/man
maps rbldomains =
message sizelimit = 0
mydestination = $myhostname,localhost.$mydomain,localhost,mail.intrix.org,intrix.org
mydomain_fallback = localhost
myhostname = xserve0.intrix.org
mynetworks = 127.0.0.1/32,64.193.94.128/28,75.0.0.0/8
mynetworks_style = host
newaliases_path = /usr/bin/newaliases
owner requestspecial = no
queue_directory = /private/var/spool/postfix
readme_directory = /usr/share/doc/postfix
recipient_delimiter = +
sample_directory = /usr/share/doc/postfix/examples
sendmail_path = /usr/sbin/sendmail
setgid_group = postdrop
smtpd clientrestrictions = permit saslauthenticated permit_mynetworks reject rblclient zen.spamhaus.org permit
smtpd enforcetls = no
smtpd pw_server_securityoptions = cram-md5,gssapi,login
smtpd recipientrestrictions = permit sasl_authenticated,permit_mynetworks,reject_unauthdestination,permit
smtpd sasl_authenable = yes
smtpd tls_certfile = /etc/certificates/Default.crt
smtpd tls_keyfile = /etc/certificates/Default.key
smtpd use_pwserver = yes
smtpd usetls = yes
unknown local_recipient_rejectcode = 550
virtual mailboxdomains = hash:/etc/postfix/virtual_domains
virtual_transport = lmtp:unix:/var/imap/socket/lmtp
xserve0:~ root#


Thanks