By default an mlalias will accept email from anybody, even from a non-CSE address, which can lead to undesirable emails being sent to the members of the mlalias. If the level of undesired emails becomes annoying the/an owner of the mlalias will have to become a moderator (see Mlalias moderation) but if the mlalias is reasonably high-volume then moderation can become onerous. To lessen the number of emails requiring explicit attention by the moderator authorised posters can be setup. Emails sent to the mailing list by an authorised poster go straight to the list without having to be approved by the moderator.
The moderator and all other owners are authorised. A non-CSE address can be added as an owner but it must be fully specified and will appear in the 'authorised poster' field rather than the owner field when the mlalias is viewed. mlalias <list> -A fbloggs -A firstname.lastname@example.org.
% mlalias <list>
authorised posters: email@example.com, @PGResearch
If an owner adds the members_can_post flag then all the addresses on the recipient list are authorised. mlalias <list> -f members_can_post.
In the above cases you know exactly who the authorised posters are. You can make life more exciting by adding a netgroup as an owner. A netgroup is a UDB class and is identified to mlalias by putting @ in front - the @PGResearch above is an example. Anybody who is a member of the UDB class PGResearch or its children is allowed to post to that mailing list. To list these people use acc PGResearch/ | grep user. You should always use the smallest netgroup that includes the people you want, e.g. @CSE_Employee rather than @Employee, as you may be surprised at who's included in the larger groups.
If an authorised poster starts sending undesirable email to the list it can be difficult to block them. If they're explicitly listed as an owner or authorised poster they can simply be removed by another owner, but if they're a member of the list then the owner has to choose between deleting them or removing the members_can_post status. If they're a member of the netgroup the owner has no power to delete them (since this requires changing their account) and may have to resort to removing the netgroup and relying upon explicit moderation until things cool down.