Additional scripts, known as "Lyris extensions" can be run automatically by Lyris, before a posting is allowed through. This is the "run a program before
in the list settings.
For example, suppose that your mailing list is a tech-support forum, and has two tiers: a free service where people can read but cannot contribute; and a for-fee service where people who have paid
are allowed to post their questions. You could write an extension script which checked the payment-status of the person posting a message, and rejected the message if the person had not paid for the
right to post messages.