In the past they sent this:
Quoted from this thread.
Unfortunately they got many complaints about that email. (read the quoted thread, and others here.)
Now apparently they don't even tell you that much.
I've said it before and I'll say it again. Dreamhost's email sucks. Dreamhost has even admitted that before in this forum (although I can no longer find that post).
Your best bet is paid 3rd party email. I say paid because every free service on the planet is either too restrictive, too intrusive, or is used by spammers (thus earning it mail blocks just like dreamhosts). I moved my email outside dramhost 2 years ago, pay $4.00/month or $48.00/annually, and could not be happier.