Can sales invoices be emailed automatically?
No
See:
I’ve just found the recurring transactions ie sales invoices so it looks as though I can.
No, you cannot. Recurring transactions involve reminders to create repeating transactions. You still need to take positive action to create the transactions. And there is no integration with emailing. Read the Guides.
I’ve set up a recurring sales invoice schedule, then just get a reminder to email it when it’s created. Would be awesome if it could send itself out automatically though.