So the ISP is right. The topic reply notifications are not sending a valid "To:" header.
I notice that the topic reply seems to use BCC - is that so the list of email addresses is not seen by anyone? Is there a way to put a dummy email address in the "To:" header so pedantic mail servers don't block these emails?
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):
Joined: Jun 29, 2004
FYI, I have same on all my 9.2.1 sites. And below is what I just received from here. I've protected the From:, Return-path: and the Envelope-to:, but the To: was blank before editing.
Subject: Topic Reply Notification - Update to my BBCode Hack (v6)
From: CPG DragonflyTM CMS <firstname.lastname@example.org>
Date: 15/05/13 03:11
Envelope-to: xxxxxxx @ xxxxxxxx.xxx
Delivery-date: Tue, 14 May 2013 19:11:09 -0700