I think you may have hit my stupidity on the head!
Now that I check the emails from Dreamhost, I see that the list starts with "Users" - I missed these, thinking that they were like the rest of the list, just Mailboxes.
I can't check this, but I guess that's where the files are - but I didn't download them, because I didn't need to backup my mailboxes. D'oh!
And thanks for pointing me in the right direction. Wish I'd known a few months ago!