Creating a working relay between two Exchange Servers / SMTP problems

Hi,

We have recently taken over a support contract for a company that is running two Exchange servers, one is running Exchange 6.5 and the other is running Exchange 2007. Their are 5 domains in use on both Servers, one domain is solely used on the Exchange 6.5 and 3 are solely used on the Exchange 2007. The 5th domain has some mailboxes set up on Exchange 6.5 and others set up on Exchange 2007. The MX record for this domain points to the Exchange 6.5 machine and I assume that any mailboxes encountered that reside on the Exchange 2007 machine are relayed to the Exchange 2007 machine. This was working, until the public IP address was changed on both PC's. Emails are still being received and relayed to the Exchange 2007 machine, but emails sent from the domain in sole use on the Exchange 6.5 machine are bounced back to the sender with the error "#5.7.1 smtp;550 5.7.1 Unable to relay for <email address sent to>". If I change the SMTP connector on the Exchange 6.5 machine to forward through the ISP SMTP server (smtp.eclipse.co.uk) then the emails send fine, but the relay between the Exchange 6.5 machine and Exchange 2007 machine stops working.

Can anyone offer any clues as to why this is happening?