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. Maybe they signed up with a personal email but are trying to send from a work email, for example.
Also, if your Subscriber's ISP had delivery issues, it's possible your subscriber was removed due to excessive bounces.
In either situation, including instructions let's them know how to proceed.