This seems to be due to the fact their email account is at Yahoo. We never had the problem before. What do you advise us to do ?
They should use another email provider.
I'm sorry for having to provide this "solution", I would normally let people use whatever system they prefer, but in this case, it's "too broken". Their email provider (Yahoo!) states that their users won't send emails other than directly (which includes that they won't use mailing lists). Given that these customers *do* want such uses, either they get their provider to change their ways, or change providers.
The other email providers are simply following Yahoo instructions when rejecting the emails from yahoo users. Gmail is slightly nicer to them by "only" sending it to the spam folder (which can be avoided with a filter), but it still means that most recipients won't receive it.
Regards
PS: AOL mail has exactly the same misconfiguration.