We are having the same issue.

We use QB online for 8 years now, all of the sudden invoices appear only 'sent' but our customers don't receive them. We resend the invoices several times on different dates, but they never makes it to the customer nor do we receive the copy which we usually do.

That happens not with every invoice. We are working with many drop ship customers so they get multiple invoices a day from us, and only some get delivered, some of them don't. The email address is always the same (saved in QB, not manually entered). 

We are struggling with this issue for around 2 months now.