It is still happening to me too.

I chatted with support about it today, again.

I was told the engineers are aware and are working on it.

I used to be able to resend it a couple times and eventually it would not bounce back.

This week, no matter how many times I resend, it bounces back.

However, I called the a few of the customers who's invoices bounced back and they DID receive it... multiple times! So, even though the QBO reports that the email bounced... it in fact did deliver.

To further confuse the situation, my settings are set up so that I am copied on all outgoing invoices, yet I did not receive the email that I confirmed with the customer who did.

Right now, I can't trust it... perhaps for now I will PDF the invoice and send it outside of QB.