Same issue here... appears it started around 4/1. I am showing 16 invoices have bounced back since then. Invoices that have been sent each month to the same email address for a couple of years suddenly are bouncing back. I didn't even notice it was happening until now as I only now started to follow up on past due invoices that apparently were never sent. In general, it would be nice if there was a more obvious message that emails are bouncing... especially when there's a huge issue like this.

I know it's not a problem with their email because if I send it directly to them, they confirm receipt. 

Tack this onto the "tabbing issue" QBO is having right now, and it's pretty frustrating.

I'll add myself to the affected users... just wanted to vent.