Ask HN: Best third party spam filtering?

We and our customers, for many legacy reasons are stuck with using Rackspace Email as our email solution. It works fine, but has the most atrocious spam filtering. Despite over a decade of complaining, Rackspace never fixes it. It sends obviously legit emails to spam (like Amazon order confirmations) and delivers the most obvious spam messages to the inbox.

We'd like to turn off their spam filtering and rely on a third party solution like https://www.cudamail.com/. We don't know anything about these services and I found Cuda Mail just by Googling for "Outsourced Spam filtering service"

Has anyone here used such a solution and, if so, which one seems to be the "best" that members of this community might recommend?

Aside - we can work around the problem for our own internal emails by simply forwarding emails to Gmail and relying on Gmail's filtering, however, our customer base is VERY un-savvy technically and we'd like a third party solution to simply solve this problem for our less technical users.

Any help or recommendations would be much appreciated. Thanks in advance!