It is probably easiest to understand how this system works by tracking a message from the mailing list to the newsgroup and then a message posted to the local newsgroup (gated to the mailing list) and see how they are handled.
Mail from the mailing list is sent to all subscribed mail addresses. A special mail alias is subscribed to the mailing list in question and so all traffic to and from the list is sent by the list server to this address.
When mail from the mailing list arrives at the local machine, the mail
alias pipes the incoming message to mail2news.pl. The mail alias
also specifies the destination (local) newsgroup.
The mail2news.pl script processes the message, applying news
headers and then uses rnews or inews to post the message to
the newsgroup.
The local newsgroup is set up as a moderated group, as this allows us to take advantage of the email capabilities of innd. Any messages posted to a moderated group are not immediately submitted to the group. Instead, messages are emailed to the moderator of the group.
By declaring the moderator of the local newsgroup to be mailing list
address, all locally posted messages to the newsgroup are automatically
mailed out to the mailing list by innd and only appear once they
have been received back through mail2news.pl which adds the
necessary 'approved' line to the messages and are thus acceptable to
innd for posting to the newsgroup.