Exchange 2007 - Bouncing back

Hi Team,
when i am sending email to few companies i am getting bounce back email.

when i checked in SPAMHUS database, it was not listed, and i checked using " MXTOOLBOX" IP was not listed anywhere. but i am getting bounce back email when i send few company users.

below is NDR report.

Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.
The following organization rejected your message: VA3EHSMHS005.bigfish.com.
Sent by Microsoft Exchange Server 2007
Diagnostic information for administrators:
Generating server: ht02.hos.local
mi.t2n@consulting.in
VA3EHSMHS005.bigfish.com #550 5.7.1 Service unavailable; Client host [124.23.22.33] blocked using Spamhaus Blocklist, mail from IP banned; To request removal from this list see http://www.spamhaus.org/lookup.lasso. ##
Original message headers:
Received: from [192.168.21.182] (124.23.22.33) by ht02.hos.local
(192.168.30.87) with Microsoft SMTP Server id 8.1.393.1; Tue, 1 Nov 2011
17:25:25 +0530
Message-ID: <4EAFDE4C.4010300@company.com>
Date: Tue, 1 Nov 2011 17:25:56 +0530
From: Stefano <stefano@company.com>
Organization: company
User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:7.0.1) Gecko/20110929 Thunderbird/7.0.1
MIME-Version: 1.0
To: =?UTF-8?B?J01paGFpIFTEg3TEg3Jhbic=?= <mi.t2n@consulting.in>
Subject: Fwd: RE: Con
References: <01ba01cc988b$76066b30$62134190$@com>
In-Reply-To: <01ba01cc988b$76066b30$62134190$@com>
X-Forwarded-Message-Id: <01ba01cc988b$76066b30$62134190$@com>
Content-Type: multipart/alternative;
boundary="------------070103070009030702030608"
Return-Path: steno@company.com
Reporting-MTA: dns; ht02.hos.local
Final-recipient: RFC822; mi.t2n@consulting.in
Action: failed
Status: 5.7.1
Remote-MTA: dns; VA3EHSMHS005.bigfish.com
X-Supplementary-Info: <VA3EHSMHS005.bigfish.com #5.7.1 smtp;550 5.7.1
Service unavailable; Client host [124.23.22.33] blocked using Spamhaus
Blocklist, mail from IP banned; To request removal from this list see
http://www.spamhaus.org/lookup.lasso.>;
X-Display-Name: =?iso-8859-2?Q?'Mihai_T=E3t=E3ran'?=