I am a server administrator for ServerMatter and I have dug in on this issue for our clients.  Since we host email AND we use QuickBooks online our own clients were saying they never got our invoices.  That's a real problem.

However, since we actually host the email for some of those clients,  we are able to see both sides of the coin.  So we created a bogus email account on our server a bogus Invoice on QuickBooks Online which we them emailed within QuickBooks and as expected - never got the email.  

We figured out why and hopefully it will help some of those on this post.

In case Intuit will not allow the post of the video, you can searching youtube for the words servermatter QuickBooks spam.  The video is here:  https://www.youtube.com/watch?v=7DjBV2iB48g