Hi, my company is Initial Technologies Ltd, we are a provider of IT systems and telecommunications attempting to implement Zimbra - first for ourselves, then if it proves successful, for our large customer base of businesses.
During the installation process i have gained a reasonable grasp of Linux, however it is most likely the correct DNS settings that have eluded me. Internal emails, calendaring and tasks work excellently and we find it very promising as a replacement for Exchange for our customers - the problem comes when sending mail to SOME external domains - hotmail works fine for example but our other email domain (initialtechnologies.co.uk - squirellmail hosted by amenworld.com) spits the emails back with this error:
This is the mail system at host mail.initialtechnologies.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

: host
mx1.initialtechnologies.co.uk[] said: 550-Verification failed
for 550-Unrouteable address 550 Sender
verify failed (in reply to RCPT TO command)

: host mx3.hotmail.com[] said: 550
Requested action not taken: mailbox unavailable (in reply to RCPT TO
Here are the outputs of our configs:
cat /etc/resolv.conf
# Generated by NetworkManager
search initialtechnologies.com
cat /etc/hosts localhost.localdomain localhost mail.initialtechnologies.com mail

# The following lines are desirable for IPv6 capable hosts
::1 localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
cat /etc/bind/named.conf.options
options {
directory "/var/cache/bind";

// If there is a firewall between you and nameservers you want
// to talk to, you may need to fix the firewall to allow multiple
// ports to talk. See Vulnerability Note VU#800113 - Multiple DNS implementations vulnerable to cache poisoning

// If your ISP provided one or more IP addresses for stable
// nameservers, you probably want to use them as forwarders.
// Uncomment the following block, and insert the addresses replacing
// the all-0's placeholder.

forwarders {;;

auth-nxdomain no; # conform to RFC1035
listen-on-v6 { any; };

zone "mail.initialtechnologies.com"{
type master;
file "db.mail.initialtechnologies.com";
cat /var/cache/bind/db.mail.initialtechnologies.com
; Addresses and other host information.
@ IN SOA mail.initialtechnologies.com. hostmaster.mail.initialtechnologies.com. (
10118 ; Serial
43200 ; Refresh
3600 ; Retry
3600000 ; Expire
2592000 ) ; Minimum
; Define the nameservers and the mail servers
initialtechnologies.com. IN MX 10 mail.initialtechnologies.com.
mail.initialtechnologies.com. IN A
I can confirm that our ISP setup ReverseDNS when asked, and the A record i setup (i hope) points mail.initialtechnologies.com to (our wan ip). Here are the records they hold:

. MX 86400 5
. MX mail.initialtechnologies.com. 86400 10
. NS ns1.amenworld.com. 86400 0
. NS ns2.amenworld.com. 86400 0
authsmtp A 86400 0
ftp A 86400 0
imap A 86400 0
mail A 86400 0
mx1 A 86400 0
pop3 A 86400 0
smtp A 86400 0
webmail A 86400 0
www A 86400 0

cat /var/log/zimbra.log (when attempting to send email)
Jan 28 12:32:46 mail postfix/qmgr[3900]: 237A81A80702: from=, size=2249, nrcpt=1 (queue active)
Jan 28 12:32:46 mail amavis[3555]: (03555-01) FWD from -> ,BODY=7BIT 250 2.0.0 from $
Jan 28 12:32:46 mail amavis[3555]: (03555-01) Passed CLEAN {RelayedOutbound}, MYNETS LOCAL []:46984 [] Jan 28 12:32:46 mail postfix/smtp[31409]: BF1B01A806FF: to=, relay=[]:10024, delay=0.47, delays=0.$
Jan 28 12:32:46 mail postfix/qmgr[3900]: BF1B01A806FF: removed
Jan 28 12:32:46 mail amavis[3555]: (03555-01) extra modules loaded: /opt/zimbra/zimbramon/lib/x86_64-linux-gnu-thread-multi/auto/Net/SSLeay/autospli$
Jan 28 12:32:46 mail postfix/smtp[31412]: 237A81A80702: to=, relay=none, delay=0.14, delays=0.09/0.02/0.04/0, dsn=5.$
Jan 28 12:32:46 mail postfix/cleanup[31403]: 5AD941A806FE: message-id=<20130128123246.5AD941A806FE@mail.initialtechnologi es.com>
Jan 28 12:32:46 mail postfix/bounce[31413]: 237A81A80702: sender non-delivery notification: 5AD941A806FE
Jan 28 12:32:46 mail postfix/qmgr[3900]: 5AD941A806FE: from=<>, size=4464, nrcpt=1 (queue active)
Jan 28 12:32:46 mail postfix/qmgr[3900]: 237A81A80702: removed
Jan 28 12:32:46 mail postfix/lmtp[31414]: 5AD941A806FE: to=, relay=mail.initialtechnologies.com[]:7025, $
Jan 28 12:32:46 mail postfix/qmgr[3900]: 5AD941A806FE: removed
Any help would be greatly appreciated, please notify if any more information is required from us.