Why does the app send an email invoice reminder automatically to one of my clients the day after I send an invoice?

Hi yanickippersiel

The automatic reminders are based on the timeline setup in options.

Go to the Gear icon Accounts and Settings -> Sales

Scroll down to Reminders and click on the Edit pencil

From there, you can enable the various reminders and set the time in days before or after the due date.

Hope this helps!