This is how to create a split SMTP domain using Exchange 2003 and another SMTP program like Exchange, Zimbra or something like that.
Step 1. Make sure you’re MX-record is already point to your Exchange 2003 mailserver and that part is working.
Step 2. Add the new domain name to the Recipient Policies:
Make sure the “This Exchange…” is not selected! (That’s important)
Now create a new SMTP connector :
Fill the IP-adres in brackets  and select an Exchange 2003 server using Add.
Goto Address space, add the new split SMTP domain and select the Allow Messages to be relayed option.
At this point the mail which can’t be delivered at the Exchange 2003 is being forwarded to the IP adres specified. At this point you have to make a receive connector (Exchange 2010 for example) or something else.