I understand how frustrating it can be when you're unable to send email invoices to your customers, WizAccounting. 

The investigation regarding this issue has already been closed. Since you are still facing the same problem, let's ensure updating QuickBooks to the most recent release. By updating, you can stay up-to-date with the latest improvements and bug fixes available. 

After updating QuickBooks, try removing and then re-adding the email address. This process can help refresh the email settings and resolve any potential issues that may have been causing errors while sending emails. 

If the issue continues, I suggest contacting our QuickBooks Desktop Support team for further assistance. They have the necessary tools to access your account and thoroughly investigate this matter. 

For now, you may consider sending your emails outside of QuickBooks Online as an alternative solution.

In addition, you can also check out this article for potential solutions to address any error messages you encounter when attempting to send emails. This provides helpful information that may assist you in resolving the issue: How to fix "Error: QuickBooks is unable to send your email to Outlook".

The Community is always here to lend hand if you need further help.