Can anybody help? We are having ongoing problems with email addresses some of our invoices are not sending despite our clients never changing the email address

There is an ongoing investigation about invoices not being sent to clients, sarah-peet-mercu.

Our product engineers are still investigating to check the cause of the issue so they can look for a permanent fix. I'd encourage you to give us a call so we can add your account to the lists of affected users. 

Here are the steps:

1. Click the Help icon in the upper-right corner of the screen.
2. Select the Search tab and tap Contact us.
3. Enter a brief description of your concern, then click Continue.
4. Choose a way to connect with us.

As a workaround, consider downloading the invoice and then sending them to your clients outside QuickBooks Online.

You can also use this article for other troubleshooting steps in resolving the issue: 3 solutions for you when customers aren't receiving your emails.

It'll be my pleasure to help if you have other concerns. 

All will be revealed in this thread. It's been an issue since February. Causing businesses no end of problem, including direct cash-flow issues.

https://quickbooks.intuit.com/learn-support/en-uk/transactions/my-emailed-invoices-are-not-being-received-by-my-customers/00/794417