I have tracked the problem to Dreamhost using the CBL in an unauthorized manner (preventing authenticated accounts from accessing SMTP when on a blacklisted IP); excerpts from the terms and conditions of the CBL:
"Thus, by applying the CBL to your relay server for outbound email, you will be impacting legitimate email sent properly."
"In the first case (outbound email traverses the Internet to get to your relay), you MUST NOT apply the CBL to connections that do SMTP authentication (or some other technique that proves that the SMTP connection is from your customer). These will often be the NATs for wireless POPs or airport lounges or hotels. Because of this a roaming user (or us) is relatively unlikely to be able to contact the right people to get the listing fixed properly. So the user will just be very frustrated (and mad at both you and us)."
"In concrete terms: you should offer authenticated inbound email access on port 25 or port 587 (or some other port), and you MUST ignore CBL listings of connections that have authenticated. If you only allow authenticated connections on port 587 (or other non-port 25 port), you do not need to implement CBL checking on it at all."
"If, inspite of 3, 4, 5 or 6 above you still want to use the CBL in an unsupported fashion (eg: block blog, web, IRC access, block on full received line traverse, derive other blocking heuristics, or block MSA submissions), you must take full responsibility yourself for the decision.
This means that you must remove all mention of the CBL (or Spamhaus) from any error messages or communications the user may see, and direct all support questions to your own support infrastructure."
For full terms, see #6 and #7 at http://cbl.abuseat.org/tandc.html
Tech support has been amazing evasive, but let drop that there had been certain "recent policy changes". After tracking down that they are using CBL in an unauthorized manner, I made a separate request 33 hours ago, but still no reply to that. Within the next 24 hours (unless DH relents) I'm moving out of this place.