most of my internet emails are being rejected

Hi, I admin a network behind a Cisco firewall. 2 days ago I installed Exchange Server 2007. I dont have an Edge Transport Server. My Hub Transport Server is handling SMTP. Everything is configured correctly. Port 25 is open and I have configured all connectors. Antispam Agents are installed on the Hub Transport Server. My ISP had registered an MX record for us (mail.khalifa-s-b.edu.qa). There are only 2 outlook accounts configured at this point and they are used by me only for testing purposes.
My problem is that although I have been able to receive emails from the internet (hotmail, Gmail, yahoo), yet when I try to send emails to the internet I get varied errors depending on the recipient. Only Gmail received an email from me but it was considered spam. I had to retrieve it from the junk bin.
When I send to hotmail I receive this error: (even if I reply to a hotmail email)
nader911t@hotmail.com
bay0-mc12-f12.bay0.hotmail.com #550 OU-001 Mail rejected by Windows Live Hotmail for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation problems. If you are not an email/network admin please contact your E-mail/Internet Service Provider for help. Email/network admins, please visit http://postmaster.live.com for email delivery information and support ##
Original message headers:
Received: from khalifa-exch.khalifaSB.edu.qa ([10.3.16.11]) by
 khalifa-exch.khalifaSB.edu.qa ([10.3.16.11]) with mapi; Mon, 7 Apr 2008
 10:45:33 +0300
From: Nader Subaih <nader_subaih@khalifa-s-b.edu.qa>
To: nader subaih <nader911t@hotmail.com>
Date: Mon, 7 Apr 2008 10:45:17 +0300
Subject: hello
Thread-Topic: hello
Thread-Index: AciYd/pJVT34sHqARdmDsJ5uzu4OAwAC02MA
Message-ID: <72B3C8203F3BBF48A17A2E20186983D00D10EAEB6F@khalifa-exch.khalifaSB.edu.qa>
References: <BAY122-W757BA61D4FE9538FBD13BFEF30@phx.gbl>
In-Reply-To: <BAY122-W757BA61D4FE9538FBD13BFEF30@phx.gbl>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
acceptlanguage: en-US
Content-Type: multipart/alternative;
      boundary="_000_72B3C8203F3BBF48A17A2E20186983D00D10EAEB6Fkhalifaexchkh_"
MIME-Version: 1.0

And when I send to yahoo I dont receive an error on my outlook but it stays on my exchange server. The Queue viewer shows it as follows:  
yahoo.com      DnsConnectorDelivery      Retry      4      Monday, April 07, 2008 10:49:02 AM      451 4.4.0 Primary target IP address responded with: "553 Mail from 89.211.46.242 not allowed - [10]." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.

I ran out of ideas. Could someone help?




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