After emailing my invoice to customer , it states that it has sent. I then go back into outstanding invoices and it says not sent?

Thank you for notifying us of this issue, @nickysmart-iclou.

I'm here to share some updates about why you're seeing unsent invoices in your account. We have received reports about the invoices being shown as undelivered in QuickBooks Online. This has been raised to our Invoicing Engineering team for further investigation.

For now, our system reattempts to send out the emails, which are then successfully delivered. They have designed an in-product fix so that the status of the transactions changes upon successful delivery through re-attempt.

In the meantime, while the fix is being worked on, please use the option to manually download the invoices and send them from outside of QuickBooks as a workaround. 

Our engineers will send periodic updates about the issue. To receive the notification via email, please contact our Support Team. They will add your account to the notification list of affected customers.

Here's how to contact us:

1. Go to the Help menu, then choose the Search tab.
2. At the bottom, click on Contact us.
   
3. Enter the keyword Account Management and Billing in the What can we help you with? box.
   
4. On the next screen, scroll-down to How would you like to connect with us section.
5. Choose your contact option to initiate a discussion with a live agent.

We ask for your patience while this is being looked into. I'll be right here of you have other concerns with the ongoing issue for unsent emails. Leave a message below or mention me on your replies so I'd get notified. Please have a nice day.