We have already reported this to SmarterTools. As of SM 17x (Build 7188) this is still a problem. It should be fixed in a future release.

We set up a Backup MX on SM17x (Build 6956). We added a few single IPs along with a few IP ranges. When we tested, we kept receiving bounces stating "No Such User Here". After an upgrade to the latest SM 17x build (7188), much testing, and failure, we decided to break the IP range down into separate IPs. That resolved this issue.