I have a Windows SBS 2011 with Exchange 2010. It's a new server and doesn't replace any previous Windows servers.
All emails, both internal and external are delivered successfully with the exception of anyone with a btinternet address. I have tested failed addresses from other Exchange servers and they send OK so it's just this server that seems to have something against BT.
I get failure messages like this:
"This message hasn't been delivered yet. Delivery will continue to be attempted.
The server will keep trying to deliver this message for the next 1 days, 19 hours and 55 minutes. You'll be notified if the message can't be delivered by that time"
I've used MX Lookup to check for any blacklisting and all is clear.