Same issue here. We have been using QB for over 8 years now and suddenly more than half of the emails sent through QB are not getting delivered. We have customers ordering multiple times a day, some of the invoices get delivered some don't.  I was added to an investigation... for several months now I get the same lame excuse email: Use the download button and send email manually - Time to look for a different provider if QB doesn't get their IPs from the spam list.