I'm experiencing the same problem also, one thing that you can do:
1. change the name server to dreamhost
2. wait till it resolve
3. ping the address to get the IP address ( their mail server is different than the website server )
Goes through a lot of trouble but if you really need it it, then thats one thing that you could do.
I hope dreamhost would include the IP address for each server when they send the welcome msg.
PS: for the webmail, they use mail.yourdomain.com so if you set different i.e: pop3.yourdomain.com you cant login into the webmail.
you can only retrieve it from POP3 or IMAP
somebody corrent me if i'm wrong.