Cant Send out mails Using Zimbra Desktop Client

[Jeke]

Hi
I have just downloaded Zimbra Desktop 1.0.4 build 1833 win32bit. i have installed it on my laptop and would like to use it as a mail client to send and recieve my mails via our domain.
Here is how our mail system is configured:
1. we have a domain called nstc.co.zm
2.We have a server that runs Debian Ver 5.0.3 and is our mail server.This server is configured to use Iredmail.
3.Currently we use webmail called Roundcube to access our mails.
4.The server is configured to use our ISP's connector to re-lay mails.(i.e. connector such as mail.icconect.zm)
5.I have used Thunderbird and it works ok but i prefer Zimbra Desktop.When i configure the Server as follows:
Account type:IMAP
Incoming mail server:mail.nstc.co.zm port:143
Outgoing mail server:mail.nstc.co.zm port 25
Am able to recieve mails but when i try to send out mail i get the following error:
504 5.5.2 <localhost> Helo command rejected:need fully-qualified hostname

Is there somthing i have missed here that requires changing?

Thanks in advance;
Jeke.