Unfortunately I don’t think there is much that can be done here in the forum. Dreamhost can’t give you any information.
If the dreamhost customer is having problems with his email, he can contact dreamhost support thru the dreamhost panel.
FWIW, I’ve never seen an error message worded the way you have it quoted. Was it a bounce type email that came back? Or is it more like a person or auto responder is replying? Are you able to contact the person via any other methods? Or from perhaps a different email address? If so, then he or she can contact dreamhost support if they don’t understand why you would be getting that type of message.