I have done everything they have suggested, called support and spent hours on the phone with them and there are no answers.  It is extremely frustrating when I email hundreds of invoices every month and they bounce back and everyone points their fingers at each other.  Support told me that my customers needed to check their email settings.  My customers have never had a problem before so why are they suddenly having a problem now?  They are not having a problem, QBO is.  I can email them from Outlook, Gmail, or my own personal email but I can't from QBO.  I'm not a detective but fingers point to QBO.