Just a quick overview:
Exchange 2003 SP2 BE Cluster (2node active/passive), OWA FE box, 2003 bridgehead which routes mail off to Mimecast for filtering.
Just implemented Exchange 2010 SP1 RU3 to co-exist with the 2003 environment:
2x CAS/HUB servers on 2 VMware VMs configured as a CAS array
3x Mailbox servers on 3 physical servers configured as a DAG
Just test accounts on the 2010 environment at the moment.
at the moment i haven't configured the Send connectors with any hubs as the source server...so im assuming the mail is routing via the routing groups that link the 2003 <> 2010 together.
One thing im finding is that transport rules don't work, would this be because i haven't configured anything as a source server under the send connectors?