Sophos Email Appliance and Exchange CAS array with NLB

Hi all,

I'm wondering if anyone can give me a definative answer on the correct way to setup the Email Appliance routing for an Exchange 2010 environment with 2 Mailbox servers and 2 CAS/HT servers that are using Windows Network Load Balancing?

We have the CAS array virtual ip as the only listed ip under Routing->Mail Delivery Servers and inbound mail is working fine

My understanding was that I would only need to enter the same virtual ip for the CAS array load balancer under Routing->Internal Mail Hosts for the outbound mail.  However it seems to get mail out to external addresses we must add the ip addresses of all our internal mail servers, both the CAS servers ip address, and even the ip of each network interface that is being used for the load balancing.

I've read through all the guides I can find on Sophos website and Google and can't really find a clear answer on the correct way to setup the routing configuration

If anyone can give me a clear explaination it would be greatly appreciated

Cheers,

Jeremy