Here what MS says:

Exchange 2010 mailbox user sends a message to an Exchange 2007 mailbox user in the same site, :

1. The Exchange 2010 Mailbox server notifies the Exchange 2010 Hub Transport server of the new mail.
2. The Exchange 2010 Hub Transport server picks up the message.
3. The routing agent determines that the version of the Mailbox server that's the home server of the destination mailbox doesn't match its own version.
4. The routing agent locates an Exchange 2007 Hub Transport server in the local site.
5. The Exchange 2010 Hub Transport server relays the message to the Exchange 2007 Hub Transport server.
6. The routing agent on the Exchange 2007 Hub Transport server determines that the target mailbox is on an Exchange 2007 Mailbox server in the local site.
7. The Exchange 2007 Hub Transport server delivers the message to the Exchange 2007 Mailbox server.