Thank you for alerting us about this matter, galvanicmac.

Let me share some insights regarding the Audit Log. If you use multiple email addresses when sending transactions, it won't specify to which email is undelivered. You'll want to resend them one by one to identify a faulty email address. From there, you can see the reason why it's undelivered when you open the transaction.

On the other hand, let's also try to configure the server to accept QuickBooks Online mail server hostnames and IP addresses to see if you can get a copy of your sent customer's invoices. To get the list of Host Names, please refer to this article and go directly to Solution 3: 3 solutions for when customers aren't receiving your emails

If you're still unable to get a copy of the invoices, I highly recommend contacting our Customer Care Support Team. They have the necessary tools to do a screen-share and troubleshoot the issue for you. Here's a link on how you can contact them: Contact the QuickBooks Online Customer Support team

I'd appreciate it if you'd update me on how the steps work and how the call goes. I want to make sure this is taken care of.