If it was sent through another organization, the block would likely be of their servers, and not of your domain (and blocks affecting our servers would definitely not be relevant here).
Also, the mail logs, and quite possibly any bounce message probably ended up with them. Regardless of fault, they have the information at their disposal to figure out what's going on, and we don't.
Side note - please make sure that their list management practices are good, and that they comply with our spam policy (http://dreamhost.com/spam.html) - particularly the stuff under "Subscriptions".
The announcement was specific to mail sent via our discussion list system.
Well, quite frankly, that's not true.