Invoices sent aren't showing marked as sent this is for all customers new and old, nothings changed as far as I can see

Welcome to the Community, @jamesharan.

I understand how impacting this is on your business. Our Engineering Team is aware of this and is working diligently to resolve it. They confirmed that even though the status of the transactions remains undelivered, our system retries to send the emails, which are then delivered successfully.

Meanwhile, check with your customers as QuickBooks Online will send emails numerous times, and engineers have reported that further attempts are often successful and delivered.

I recommend contacting our Customer Service Department. Once connected, a live representative can safely look into your account and add your name to the list of users who have been affected. This way, you'll receive email notifications of new updates.

When your clients receive and pay their invoices, you may record invoice payments in QuickBooks Online.

Let me know if there is anything else I can do to assist you with your invoicing. I've got your back and am always willing to lend a hand. Have a wonderful day!