The next question to ask is there a common denominator with the domains that are being sent to... domains in the past that have been problematic off the top of my head are gmail, aol, yahoo, comcast, and i'm sure there are a few more.
Also you can send yourself an email from the site to determine the originating IP from the extended headers and then use a tools site such as mxtoolbox.com to check blacklists.
You have to understand that dreamhost originates your mail differently when you send from your domain via a standard mail client or webmail, than it does when a script generates mail from your webserver (specifically the originating IP is different).
If you find out your webserver is on a blacklist or 2 there are about 3 options you can work with. 1) ask support to move you to a different webserver that isn't on blacklists. 2) work with support to get blacklists cleared, 3) configure smtp to that your mail originates from dreamhosts mail servers rather than the webserver.