We have 2 Exchange CAS servers running NLB. We use an outsourced SPAM provider who forwards messages to us. All of the messages from them go to a single CAS server because we only see one IP address from them.
Is there any way to configure NLB so that SMTP connections not from our network are load balanced per connection, while all other connections are balanced by source IP?
I think you just need to remove affinity. Affinity does a hash between ip source and destination, so you must not use that.