Thank you very much to all!
I have solved the problem with your guidance?
After reading the last messages from Mark, Peter and Matt, I decided
to dig into the "list address affair (the sender header)"? for my
surprise, the email sender was "listname-bounces[at]myhostname"
instead of "listname-bounces[at]myhostname.domain_name", after
changing the option "host_name" in "General Options" to a full
qualified domain name, the problem was solved!
My ISP was discarding the messages because the sender was not
formatted properly... not a "legal" name for their SPAM filters I