We have done similar work on our end and found the same results, basically Intuit (Quickbooks) servers are being blocked due to the volume of spam being sent (spoofed of course).

Although this is difficult for Intuit to resolve because they are not in control of the spammers it can be fixed, QB need to have multiple servers and switch servers when an IP gets blacklisted, they are clearly not monitoring their server IP's for blacklisting.

I know they do have numerous IP addresses that they use but they need more. Also most IP's that are blacklisted are released from the list after a period of time so I dont understand why it is taking so long for the issue to stop for some people.

Clearly when they have responded to complaints telling people it will take x weeks to resolve the problem they are just playing for time hoping the IP will be removed from blacklists automatically, this is just sloppy!

We are at the mercy of QB with this one, it is not acceptable to just hide from the issue and not respond to their customers, they need to get their act together and deal with it.