Starting today - lost connection after DATA (0 bytes) from xxx and END OF DATA - 554 5.7.1

[Simon]

All of a sudden (Monday morning - typical) we have starting getting this error:

postfix/smtpd[4696]: lost connection after DATA (0 bytes) from XXX...

Which is resulting in the following bounce back to senders:

Server refused mail at END OF DATA - 554 5.7.1 This message has been blocked because the return email domain is invalid.(failed to obtain DNS record for domain sendingdomain.co.nz)

I have checked several times and changed DNS servers to test this but im totally at a loss as to whats happening. Ive found lots of reference to changing the MTU on the network card... but nothing that has helped me. Can anyone please shed some light on what could be causing this issue?

Thank you!!!!

Simon

Postfix is running on VMWare VM on debian

alias_database = hash:/etc/aliases

alias_maps = hash:/etc/aliases

append_dot_mydomain = no

biff = no

body_checks = pcre:/etc/postfix/body_checks

config_directory = /etc/postfix

header_checks = pcre:/etc/postfix/header_checks

inet_interfaces = all

mailbox_command = procmail -a "$EXTENSION"

mailbox_size_limit = 0

maximal_backoff_time = 2000

maximal_queue_lifetime = 1d

message_size_limit = 52428800

minimal_backoff_time = 500

mydestination = mysql:/etc/postfix/mysql-transport.cf

mydomain = newmedia.net.nz

myhostname = mx1.newmedia.net.nz

mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128

myorigin = newmedia.net.nz

queue_run_delay = 500

readme_directory = no

recipient_delimiter = +

relayhost =

smtp_host_lookup = native

smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)

smtpd_client_restrictions = check_client_access hash:/etc/postfix/amavis_bypass

smtpd_data_restrictions = reject_unauth_pipelining

smtpd_helo_required = yes

smtpd_recipient_restrictions = permit_mynetworks,        reject_unauth_destination,        reject_unknown_sender_domain,        reject_unknown_recipient_domain,        reject_invalid_hostname,        reject_non_fqdn_sender,        reject_non_fqdn_recipient,        reject_rbl_client zen.spamhaus.org,        check_client_access hash:/etc/postfix/access

smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem

smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key

smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache

smtpd_use_tls = no

transport_maps = mysql:/etc/postfix/mysql-transport.cf

unknown_local_recipient_reject_code = 550

virtual_alias_maps = mysql:/etc/postfix/mysql-aliases.cf

[Wietse Venema]

Simon:

> All of a sudden (Monday morning - typical) we have starting getting this
> error:
>
> postfix/smtpd[4696]: lost connection after DATA (0 bytes) from XXX...

Your server loses the SMTP connection.

> Which is resulting in the following bounce back to senders:
>
> Server refused mail at END OF DATA - 554 5.7.1 This message has been

> record for domain sendingdomain.co.nz)

That does not come from Postfix. It lost the connection, remember?

Someone put a "police agent" system between your server and the internet.

Wietyse