You can manually send a reminder message about a due invoice by e-mail, or you can set up the system to automatically send due invoices by email to clients.
Details of how you can manually send the message can be found here: how do I send an invoice by e-mail.
Automatically due invoices sending
From the ‘Configurations’ section, you can set up automatically sending due invoices by email.
You can set how many days before / after the due date to begin sending and if / how often to resubmit until the payment. You save much time because you no longer have to send manual notifications and your clients will pay faster because it will be impossible to overlook the invoices sent by you.
You can configure the message to be sent automatically. Read more here.