Dreamhost uses different email servers. There are two approaches you can use.
Scroll to the section titled: Connecting directly to your DreamHost mail server. and use that to determine the hostname for your mail cluster. The user portion of your credentials needs to be your full email address.
If your domains DNS is hosted elsewhere, and you want to use DNS entries for the domain that still exist in dreamhost nameservers you can change your local system to use dreamhost nameservers to use 126.96.36.199 and 188.8.131.52. (Don't leave them like that when you're done...switch back to your isp's or googles or however you normally have your local system.)