incoming mail with communication between exchange 2003 and 2010

Hi there,

Since 2005, my company have one exchange 2003 on Windows 2003 Server. We use asso.local as domain. The format of the company is name@asso.org.

Recently, I decided to create a new forest/domain newasso. Local on Windows 2008 and Exchange 2010 environnement. IT is installed on recent and efficient hardware.

Now, I have one organization :
-      User uses mail with format as name@asso.org
-      One forest on Windows 2003 and Exchange 2003
-      One other forest on Windows 2008 and Exchange 2010
-      Incoming mails arrived on exchange2003

If I have well understood, Exchange 2010 uses cas for incoming mail and not smtp.
I need to have the both server  as the same time for 2 months.

If one mail is not for one user that is not on exchange 2003, I would like that mail is follow and manage by Exchange 2010. Please help me  

Sorry for my english !

Thanks,

Regards,

Simlip