I've tried sending an invoice 3 times via email but it hasn't been received by the client yet and I've tried a different email?

Welcome to the club unfortunately.

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

I’ll share some information with your concern about sending invoice in QuickBooks Online (QBO), robsonaerials-ao. 

Currently, there’s an open investigation (INV-79014) regarding this matter. Thus, our engineers are still working on a permanent fix for this issue.

With this, I'd recommend contacting our Support team, so you'll be added to the list of affected users. To do so:

1. Go to the Help icon at the top to select the Contact Us link.
2. Enter a ''customer does not receive the sent Invoice'' box and click the Continue button to see the list of support options.

I’m adding some links below for additional resources. These articles will guide you on how to troubleshoot email delivery issues. They’re from another region, but the solutions also apply to the UK version.

• How to view list of bounced emails with delivery errors.
• Delivery Errors when trying to email a sales form.

Additionally, you can bookmark this link in your browser for future reference. It outlines the steps to resolve when customers are unable to receive your emails.

We appreciate your patience as we're working on this. Please let me know in the comments down below if you have any other questions aside o this customer does not receive the sent Invoices.I'll be around to help you again