Hi there, @TopTrousers.

Thank you for reaching out to us and providing extra details. I can share some insights about emailing invoices in QuickBooks Online.

After sending the invoices, your customers have the option to acknowledge it or reply. You can also confirm if they have already viewed the transaction.

The system will recognize that an invoice has been viewed when the customer clicks on the Review and pay button. The customer's name and time viewed will reflect on the activity section.

1. On the email transaction, click Print or Save.
2. Go to the Activity tab. It shows when the customer viewed the transaction as well as the messages you've sent.

On your QuickBooks, here's how to check the status of invoices sent via email:

1. Click Sales.
2. Choose Customers.
3. Select the name of the customer you want to view.
4. Open the invoice and go to the Activities section.

For in-depth details, please check this article: Use e-Invoice.

In case you're unable to email invoices in QuickBooks Online, you can send the invoice outside QBO. To report this, you can start a conversation with our support to provide your call back number. They can add you to the list of affected users.

1. Log into your QBO account.
2. Click the Help tab and choose Contact us.
3. Enter unable to email invoice and click Continue.
4. Click Start messaging.

That should guide you through the right path, TopTrousers. Let me know if you have additional questions about e-invoicing. I'm still here to provide answers. Have a good one.