I had similar problems with AOL and a mailing list system I set up for my college alumni. It simply retransmits messages to everyone on the list, all as bccs, with a phantom address shown as the "to". AOL quietly dumped the majority (but not all messages), so the recipients didn't know they were missing anything until they received copies of replies to messages they hadn't seen.
AOL was not in the slightest bit interested in the problem. I solved it by setting up a separate forwarder on the server for each AOL user (quite a "faff").
That worked, but I did suggest to them that they get a "proper" ISP. Most have meanwhile done so.