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Thanks for bringing your concern to the Community, charlene-airconc

Intuit uses its official email (quickbooks@notification.intuit.com.) when sending invoices to your customers. Because of the origin of the return email address, some email providers mark it as spam.

​You can suggest your customers set up their emails and allow intuit emails to go to their inbox. 

Have them follow the steps available in this article. The steps both work with QuickBooks Online (QBO) and QuickBooks Desktop (QBDT): Receive important email messages from Intuit.

Another option for QBO is to let your customers check their junk mail and spam folders.

1. Go to the Gear icon.
2. Under Your Company, select Account and Settings.
3. In the Contact info section, select Email.
4. Clear the Company email field and enter the desired address, even if it appears correct as is.
5. Select Save and then Done.
6. Send yourself a test transaction.
7. If you still don't receive the email, try using a different email address that has a different domain.

For detailed instructions, you can check the solutions when customers aren't receiving your emails: When Customers Aren't Receiving Your Emails.

If they can't still find the email, you can turn on the  Email me a copy feature. This will let the system automatically send you a copy of the email that you've sent to your clients.

Let me guide you on how:
 

1. Click the Gear icon in the upper right corner.
2. From the Your Company column, choose Account and Settings.
   
3. Select the Sales tab on the left.
4. In the Messages section, click the Edit icon (pencil image).
5. Put a checkmark in the Email me a copy at [your email address] box.
   
6. Click Save and Done once you're through.

I recommend reading this article that will guide you in personalizing your form designs and adding the information that matters the most to your business: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Our doors are always open to help you again if you need further assistance sending invoice reminders. Have a great day.