Once invoices are sent out to clients, you can set late payment reminders that will be sent to clients automatically.
In order to do that you need to click on your name (bottom left) → Company settings → Project accounting → Invoicing → Late payments reminders. You can set 3 late payment reminds, which will sent a reminder after a certain number of days after the due date.
When a reminder is sent out to the client, you will receive a copy as well. The email is sent to the email address you've set in the Company settings area.