How can I prevent outside mail in Mailing List

Hello, I have 2 mailing lists and set as private. We want these groups to message only inside, that is, when mail comes from mail with extensions such as outlook, hotmail, gmail, we want not to forward this message to the users included in the mailing list. for example, we have zxc.com domain, we want to receive mail for these mail groups only from mail with zxc.com domain. I hope I was able to explain, I am waiting for your support. Thank you.