I'll provide different information and steps about clients that are receiving reminder e-mails to pay their invoices.
Since you've confirmed that this has never happened in the past, a user could have sent it manually. To verify this, you'll need to visit the Audit log page. From there, you're able to review the history of the reminder e-mails.
Click the Gear icon in the upper right corner.
Select Audit Log.
Select the Filter panel to filter User, Date, or Events and narrow the results.
Once verified that the invoice reminder is sent by System Administration, this has been a recently reported issue. Our product engineers are now working to fix this as soon as possible.
I suggest contacting our QuickBooks Support Team, so they can add you to our notification list. This will help our engineers determine the number of affected users. You'll also receive an update through email once this has been resolved.
Here's how to contact them:
1. Open your QuickBooks Account.
2. At the top right, click the Help icon.
3. Click Contact Us.
4. Under What can we help you with?, enter your concern about clients are receiving reminder e-mails.
5. Click Let's Talk.
6. You'll be routed into the Choose a way to connect with us page.
7. Select how you want to contact them and fill in that information.
However, if you verified that no one from the user or system admin set up the reminder e-mails, I'd suggest going to the Account and Settings section. That said, it might be the Automatic invoice reminders feature is turned on. That is why your clients are automatically receiving reminder e-mails to pay their invoices. Please click this article to see additional information: Automate invoice reminders in QuickBooks Online.
On the other hand, to make sure that your clients are only receiving legitimate e-mails concerning their invoices, they need to check if the email address used is from Intuit. You can open this article to see several different email addresses that are from Intuit: Is this email I received a legitimate communication from Intuit?.