To only receive emails from your subscribers to your Substack email address (firstname.lastname@example.org), follow these two steps.
1. Head to your Settings page and scroll to "Receive email replies to your posts from".
2. Select "All subscribers" from the dropdown box.
Why would I change my settings?
If your email replies setting is currently set to "Everyone", then anyone can email you at your @substack.com author alias.
There have been instances of email alias spoofing in which a spammer can send a message using your @substack.com author alias (to a list of non-subscriber addresses the spammer has acquired outside of Substack). If you have your setting set to "Everyone", you may receive email replies from non-readers to a spam email that was sent by a spammer.