Hi Kevin,
Once you enable the Auto sender on the Sales>Email>Settings menu this will opt in all the customers you have on your account (at that time) by default.
Unfortunately at the moment there are no global opt in/out settings after this initial step. You can only opt in/out customers manually using the guide below:
https://www.clearbooks.co.uk/support/guides/email-sales/customer-email-reminder-settings/
The Auto sender setting itself will send an email to a customer as soon as an invoice has been created to let them know just that.
You can create a recurring template on the Sales>Invoices>Recurring menu. This template will generate a new invoices each week/month/year/etc. Once a new invoice is generated an email will be sent to your customer automatically as long as the Auto sender is enabled.
https://www.clearbooks.co.uk/support/guides/recurring-invoicing/how-to-create-a-recurring-invoice/