What happens when an unknown person tries to post to my List?
All incoming email messages are verified against your list of approved Subscribers.
Emails from unknown addresses will have a rejection message returned. This rejection contains instructions on how to be added to your List.
Why would the rejection have instructions to be added?
Sometimes, your Subscribers may be emailing from a different address. For example, they signed up with a personal email but are trying to send from a work email.
In either situation, including instructions in the rejection message let's them know how to proceed.