I've successfully installed Exchange Server 2010, and I've been able to move mailboxes from my existing Exchange 2007 to the new one. However, I'm not able to send e-mails from 2010 to 2007. On 2010, I can send and receive externally through the Internet, and I'm able to receive internally. I just can't send out to 2007. I've read the following thread and tried the solution.
What happens is that the mail gets stuck in the queue and it dies out after 2 days. I've disabled all the receive connectors except for the Default and Client, and it still doesn't work.