Quickbooks knows what the issue is as they have been told in at least one other thread on this issue.
Three IP addresses used to send invoices are blacklisted on SpamCop. Therefore most servers will bounce emails received from those IP addresses. Other IP addresses used by QBS are not blacklisted. Hence why some invoices get sent and others don't.
The reason for the blacklisting is phishing emails being sent from those IP addresses in the name of Quickbooks - so they are correctly being blacklisted.
The issue has been known for at least 4 months.
It is pretty scandalous that a large business like this has been unable to rectify the problem for so long.
Here is the immediate answer - stop using the affected IP addresses?!
