I was thinking about this, and I can get the ip address of the mahine for you, or you can. We don't need support!
If you're familiar with SSH then you can log into your server by going to servername.dreamhost.com. To find the name of your server log into the panel > Billing > Manage Account. You'll see a list of the users you have set up, if they have shell access and what server it's on.
Once you've got shell access you can just ping webmail.yourdomain.com to see it's IP address. All of the dreamhost servers check the DH DNS for a listing first. So even though your domain is expired the entry will still be in the DNS system. Of course you can also use command line programs to check you're mail, but I'm not familiar with them off hand, though I'm sure there's something in the wiki about it.
If you're not familiar with SSH I can just ping your domain name for you and get it that way.
Or perhaps better yet you could log into the Panel > Mangage Domains > add new domain/sub-domain. Add a mirrored domian of dreamhosters.com of webmail.yourdomain.com. Give that a bit to go through, as little as 5 min as much as 4 hours. Then you should be able to just go to the subdomain you created and access your webmail install. I suspect that you could mirror mail.domain.com as well if you'd prefer to use an E-mail client (the host file method will work for your E-mail client as well)
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