(optional) Defines whether to allow Visitors on your mailing list. Allowing Visitors means that the web interface will allow non-members to Read and Search the past discussions on your
mailing list. They will also be able to Get Info about your mailing list. A Visitor cannot contribute to a mailing list, and cannot receive mailing list contributions via email.
Allowing Visitors is often a good way for prospective member to get a taste of what a mailing list is about. They will be able to read past postings to see if the mailing list interests them. If it
does, they can apply for membership.
Allowing Visitors is also useful for specialized mailing lists, where the information is not confidential, but you do not want just anyone joining. For example, a mailing list discussing Internal
Telecommunications standards would want outsiders to be able to read the discussion but, you may want the mailing list to be Private, so that only members of the standards committee can
belong to the group.