As a spam stopping measure, any message sent to your list from an email address that is not subscribed will come to you, the moderator, for approval. Here is an example of an email sent to the moderator of the webchat discussion list.
You click on the link following, "At your convenience, visit". Your link will be the identical, except you need to replace webchat with the name of your list:
Enter your password and log into the Moderator's Panel which shows you what you need to approve: new members and messages from non-members of the list. For each item select either Accept or Reject. After you've handled all the messages, click Submit All Data.
If you approve it, a message is sent to everyone in the group. If you reject it, a notice is sent to the sender.