The Mailing list table is a standalone table with no connection to any other parts of the database.
It is simply used to record any enquiries you might have but you've nothing available for rent at that time.
To add a new record, click on "Mailing List" underneath the letting unit category in the sidebar then click the "Add New Mailing List" button in the top right.
Enter the contact details of the mailing list enquiry and choose their preferred property type (you can select multiple options), minimum bedrooms, maximum price (per month) and whether a garden is essential or not.
Notes can be added.
Whenever you have a property become available for rent, you can check the mailing list table and send out links to any suitable property you might have available.