Thanks for contacting DreamHost Support! My name is Nate and I am the Abuse/Security Supervisor here at DreamHost. We are as concerned about these constant blocks as you are. Unfortunately, our investigations have turned up nothing so far. Other major providers are not blocking the discussion servers, which you would assume would happen if there was actual spam. The discussion list servers are all clean on all of the public blocklists.

Part of the problem that we are running into is that AT&T isn't giving us any context for the blocks and, given how the Mailman program that we use for discussion lists is structured, there is simply no way to know anything about any of the mail being sent out without being provided evidence of it. Mailman only logs messages if the discussion list owner enables it.

AT&T's system is entirely automated. We ask for more information every time and every time we get the same message back, regretting any inconvenience they may have caused. All attempts to request further information have been ignored.

Without proof or context (and given the lack of blocks from other providers), our current working theory is that there is something in AT&T's anti-spam algorithm that is sensitive to the volume of sending that comes from discussion list servers and it's not looking at the context of the email very well.

I'm sorry that I don't have more for you here, but we increasingly believe these to be unwarranted and unjustified blocks.