We have been having the same issue since last week. We have also tried all the troubleshooting steps given to us by the QB helpline but without success. Our IT department have looked at it and have come back to us with the same information as Larisa's hosting company i.e. QuickBooks will need to get their IP delisted from the Spamcop blacklist before the issue can me resolved. 

We're also hoping this isn't going to take much longer for QuickBooks to resolve.