There are three kinds of basic lists (determined by the setting of Allow Postings By on the List Security page):
You can further restrict the message posts with:
A password requirement.
A posters' list.
Those who send mail to a list restricted by a password and/or posters' list will have their mail returned with a "Restricted Post" message.
Note: If there are only a few individuals who the moderator wants to allow to post to the list, the moderator can give those few the appropriate password. However, if there are more than a few individuals who are permitted to post to the list, it may be more efficient to use a posters list.
Creating and Managing Lists