I have a suggestion that I want to share so we can fix issues with your emailed invoices, Christopher.

Sometimes, you may not be receiving emails and aren't being notified. If this happens, it's most likely because your email service provider's server doesn't  provide us a notification of the delivery error. 

However, in your case someone is already intercepting your invoices and estimates and even pretended to be as you. I recommend reaching out to an IT Guy to help you double-check and configure some of your software applications, servers, and databases. This will prevent hacking issues when using QuickBooks Online (QBO).

Furthermore, learn from this article on how you can fix some of the error and send transactions with attachments: Fix The Delivery Server Down Error In QuickBooks Online.

Let me know if you have any other concerns  about bounced emails and delivery issue. I'm always right here to help.