Windows 2000 IIS5 SMTP Problem

[TeleMaster]

Our company send e-mails to a large corporate ISP.
The e-mails are being bounced with the following error code:
Reporting-MTA: dns;bossrv01.telemessage.com
Received-From-MTA: dns;bospdc
Arrival-Date: Sun, 3 Aug 2003 15:37:06 +0000

Final-Recipient: rfc822;***@***.com
Action: failed
Status: 5.7.1
Diagnostic-Code: smtp;550 (some times it is 530) 5.7.1 unknown host or
domain: ***@***.com

I made sure all DNS records of our server are OK (NSLOOKUP and R-Lookup)
I have also check that it is not in the open relay list or spam list.
When I am simulating an SMTP session via telnet to port 25 from the same
originating server ('helo / Mail from: / RCPT to: / data' etc.), the message
goes through without an error.

For the record I must add the following fact.
The destination domain record and the actual server that is responding are
not identical (i.e. MX record for the domain is aaa@bbb.com but the actual
server that replies on the other end is ddd1.eee@fff.net or
ddd2.eee@fff.net)

Server: Windows 2000 Server + IIS 5 Service Pack 4 installed.


P.S: I have added a transcript from the SMTP log:

2003-08-04 08:16:49 Destination_Domain_Server OutboundConnectionCommand
SMTPSVC1 BOSSRV01 - 25 EHLO - bossrv01.telemessage.com 0 0 4 0 10063
SMTP - - - -
2003-08-04 08:16:49 Destination_Domain_Server OutboundConnectionResponse
SMTPSVC1 BOSSRV01 - 25 - - 250-Destination_Domain_Server 0 0 35 0 10094
SMTP - - - -
2003-08-04 08:16:49 Destination_Domain_Server OutboundConnectionCommand
SMTPSVC1 BOSSRV01 - 25 MAIL -
FROM:<2306836_165637_310943@bounce.testwms.telemessage.com>+SIZE=3193 0 0 4
0 10094 SMTP - - - -
2003-08-04 08:16:49 Destination_Domain_Server OutboundConnectionResponse
SMTPSVC1 BOSSRV01 - 25 - - 250+2.5.0+Address+and+options+OK. 0 0 33 0 10125
SMTP - - - -
2003-08-04 08:16:49 Destination_Domain_Server OutboundConnectionCommand
SMTPSVC1 BOSSRV01 - 25 RCPT - TO:<User@Destination_Domain> 0 0 4 0 10125
SMTP - - - -
2003-08-04 08:16:49 Destination_Domain_Server OutboundConnectionResponse
SMTPSVC1 BOSSRV01 - 25 - -
530+5.7.1+unknown+host+or+domain:+Destination_User@Destination_Domain 0 0 58
0 10188 SMTP - - - -
2003-08-04 08:16:49 Destination_Domain_Server OutboundConnectionCommand
SMTPSVC1 BOSSRV01 - 25 QUIT - - 0 0 4 0 10297 SMTP - - - -
2003-08-04 08:16:49 Destination_Domain_Server OutboundConnectionResponse
SMTPSVC1 BOSSRV01 - 25 - - 221+2.3.0+Bye+received.+Goodbye. 0 0 32 0 10328
SMTP