I have a mixed environment. At location QBC I have an Exchange 2003 server, at location CA I have an Exchange 2003 server. Location KY I have two Exchange 2007 servers FE & BE. Server BE currently has the mailbox and hub transport role installed. The server FE had the CAS and hub transport role installed. My send connector is set up to use the BE as the only source. Users whose mailboxes are on BE cannot send any email. Every user. The can receive email from the outside world. Also, if a device or software that hits the server via a recieve connector the message is delivered.
Messages that are sent from Outlook stay stuck in the Outbox. Messages that are sent from OWA end up in the drafts folder.
If I move a user's mailbox from BE to any other server in the organization, everything works. Since that left me with no mailbox server on site I installed the mailbox role on FE and have moved some mailboxes over. Again, everything works as long as the mailbox is not on BE.