Once invoices are sent out to clients, you can set late payment reminders that will be sent to clients automatically.
To do so, click on your name (bottom left) → Company settings → Project accounting → Invoicing → Late payments reminders. You can set 3 late payment reminds, which will send a reminder after a certain number of days after the due date.
NOTE: 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.