Mail rejected from some senders after installing Exchange 2003 server

I'm upgrading my clients network to pure Windows 2003/Exchagne 2003.  I installed the Exchange 2003 server alongside their existing Exchange 2000 server.  No mailboxes have been moved yet, No firewall changes have been made.  THe only thing I've done is to start Public folder replication.   Inbound mail is still processed by the Exchange 2000 server.  Mail seems to be coming in for the most part ut when I send mail from my outside company e-mail, I get the following error.

There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
  <exchange.abcdomain.com #5.5.0 smtp;500 Syntax error, command unrecognized>

Mail from my outside e-mail worked perfectly prior to the installation of the Exchange 2003 server... THis sounds like I may have missed something but I'm a bit stuck as to where to look.
azychekAsked:
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VahikCommented:
since u have not TOUCHED the 2000 server then it is most likely ur FIREWALL....
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