I have postfix 2.5.1 running on ubuntu 8.04

Recently I have discovered that certain customers were having emails bounce back when sent to our servers and upon digging in the mail logs found that the connections seem to be getting dropped.

Feb 1 00:58:29 ds7132 postfix/smtpd[21197]: connect from snt0-omc2-s33.snt0.hotmail.com[]
Feb 1 00:58:29 ds7132 postfix/smtpd[21197]: 7FAC03181DA: client=snt0-omc2-s33.snt0.hotmail.com[]
Feb 1 01:27:30 ds7132 postfix/smtpd[21197]: lost connection after DATA (29151 bytes) from snt0-omc2-s33.snt0.hotmail.com[]
Feb 1 01:27:30 ds7132 postfix/smtpd[21197]: disconnect from snt0-omc2-s33.snt0.hotmail.com[]
This one hotmail server above is displaying these sets of log reports over and over again.
It consistently drops out at 29 minutes and then retries about 4 minutes.
All other hotmail servers connect and send first time, only this one repeatedly has an error.

I also have a number of other servers that are showing the same connect -> lost connection -> disconnect pattern.
However these other servers all have a much different timespan between the connect and the lost connection, although mostly consistent to each server.

At least 2 of them are MS Exchange servers but I don't know if they all are.

One server also displays the following message immediately before the lost connection error.
Feb 1 01:19:45 ds7132 postfix/smtpd[32697]: warning: network_biopair_interop: error reading 7998 bytes from the network: Connection reset by peer
I have contacted the system administrators of the servers in question to try to obtain the error from their logs but am still waiting.

Does anyone have any ideas or suggestions as to the cause?

# See /usr/share/postfix/main.cf.dist for a commented, more complete version

# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

# TLS parameters
smtpd_tls_loglevel = 3
smtpd_tls_cert_file = /etc/postfix/postfix.cert.pem
smtpd_tls_key_file = /etc/postfix/postfix.key.pem
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
#smtpd_tls_security_level = may
smtp_tls_security_level = may

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

myhostname = mail.ppcnseo.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = $mydomain
mydestination = ds7132.dedicated.turbodns.co.uk, localhost.dedicated.turbodns.co.uk,localhost
mynetworks = [::ffff:]/104 [::1]/128
mynetworks_style = host
mailbox_size_limit = 0
recipient_delimiter = +
virtual_alias_maps = hash:/etc/postfix/virtual
sender_bcc_maps = hash:/etc/postfix/bcc
mailbox_command = /usr/bin/procmail-wrapper -o -a $DOMAIN -d $LOGNAME
home_mailbox = Maildir/

smtpd_sasl_auth_enable = yes
broken_sasl_auth_clients = yes
smtpd_timeout = 600s
smtpd_recipient_limit = 3000
smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination permit_inet_interfaces
smtpd_soft_error_limit = 200

append_at_myorigin = no
max_use = 200

milter_default_action = accept
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891

smtp_bind_address =
daemon_timeout = 1800s
trigger_timeout = 15s
smtpd_error_sleep_time = 2s
delay_warning_time = 48
debug_peer_list = divingphi.com, phi-phi-adventures.com