Hello there, anna5.

Thanks for getting back to us. I'm here to help and provide some insights about sending a mass email to customer in QuickBooks Online (QBO). 

You should have the Microsoft Outlook installed on your computer so you can send a mass email using the "batch-email" button in QBO. The other option is, you can copy your customer's email address and send them the information using your personal email (G-mail) account.

However, if you have the Microsoft Outlook and the issue still persists after you processed the troubleshooting steps. I encourage you to contact our QuickBooks Online Support Team so they can investigate this further. 

Here's how you can contact our phone support: 

1. Go to: Get help with QuickBooks Online.
2. Click the contact us link. 
3. Select the Search for something else, and type-in Customer Support. 
4. Click Search.
5. Scroll-down and click Get Phone Number. 

Please know that you're always welcome to post if you have any other concerns. We're always here to help you out.