Remember when I talked about throttling in Exchange 2010? One of the drawbacks was that you could only configure throttling at the mailbox level, meaning if the account didn’t have a mailbox, you couldn’t control how it was throttled. This seriously affected (broke) cross forest and machine account scenarios. They were all governed by the fallback policy, which is hardcoded.
Turns out, we’re fixing this in Exchange 2010 SP1. The Ehlo blog has a really good write-up of the problem and the way SP1 fixes it. If you’ve got a MAPI program that might be running as a machine or cross forest account, you’re gonna want to read this.