Zimbra reponse with 5-10s before SMTP banner
220 mail.example.com ESMTP Postix

Need to do a trace at the server
tcpdump -s 1500  -w /tmp/server_trace.pcap

Looking at a wireshark trace, i see the server sending out “MDNS” packets which was installed along with ubuntu-desktop.

sudo update-rc.d -f avahi-daemon remove
/etc/init.d/avahi-daemon stop