I'm getting this issue as well for specific customers. I can email the exact same (copied and pasted) email address from anywhere else, but it comes back as Undeliverable in QBO. Additionally, nowhere in QBO is the actual SMTP error code displayed, so there is absolutely nothing to provide to the customer to have them check in case it is an error on their end. We've checked their spam logs and found that there isn't even a reference to an attempt at an email from QBO, or even an email around the time I tried sending the invoice. My own email account gets CCed on the email attempt from QBO, and those go through without an issue.

If the QBO engineers don't know how to fix this, at least display the SMTP error code in the Audit Logs so that this information can be relayed, and the rest of us can actually do an investigation more thoroughly.