[rubab]

I got a Exchange 2003 Server running on BES 5.0 on Domain A - this domain faces the internet

Setting up a new Exchange 2010 Server running a separate BES 5.0 on Domain B - this domain is not facing the internet and is routing traffic via the old Exchange 2003 Server on Domain A

The plan is to move user mailboxes from Domain A to Domain B

I've created a new user on Domain B - this user resides on Exchange 2010 and can send and receive emails to the internet as traffic is routing via Domain A old Exchange server - however when I try to setup user's Blackberry on Domain B - the BES can see the user and generate activation code - which is being received by the user but handset cant see it as its searching for the user on Domain A

Both Domain A and Domain B will be hosting users for the same domain name

[knottyrope]

did you move the BESAdmin mail box yet?

follow this doc or install new BES on 2010 and use transporter to move users over
KB22601-How to migrate from Microsoft Exchange Server 2003 or 2007 to Microsoft Exchange Server 2010 in a BlackBerry Enterprise Server 5.0 environment