Twenty years ago this was all the rage, and precisely why Google and Yahoo moved away from the model with their Groups products.
Discourse has a mailing list mode which emulates some of the features seen on products such as ListServ, but it’s not ListServ. Anyone looking to substitute an old mailing list needs to understand the differences beforehand and educate their users, otherwise this kind of problem is inevitable. It’s not a bug, just a consequence of trying to use one product as if it were a carbon-copy for another.