I set up google apps as the email provider for one of my domains and after adding some users realised they could send mail to each other but not receive it from addresses at any other domain than mine.
I checked the MX records were set up correctly (they were, and had been fro more than 48 hours), and asked Dreamhost support who weren't interested as I was using google.
The fix was to go to Manage Domains > Refresh. I was then able to use the accounts fully. Hope this helps someone!