At least one ip (18.104.22.168) is listed on UCEPROTECT
I wonder if they are getting hit because all their shared clients send emails via only a few ip addresses?
A few people sending what people consider spam could be screwing them up, if their mail volume is that big it might be difficult to track down.
Microsoft Office 365 has an option to report spam/phising, I assume lots of people have just reported emails from those ip addresses as spam.
Edit: Scrap that their Status page shows what appears to be hosts themselves being blacklisted rather than a central server.