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.