You say you're going to fix this issue yet 2+ months since your post and over a year since this original thread was made Intuit has STILL not fixed this. I'm having the same issue with QB saying it can't reach our customers. This is the message it says: "We can’t reach this customer. Please verify the email address with your customer or use a different email."

BS. The email addresses are correct. It's not an issue with customers' email addresses, it's not a problem with all of our browsers, it's an issue with INTUIT. 

1+ years later and still no fix in sight...