Hi everyone! I have already reported this to Rod at SmarterTools, but I wanted to also post it here in case anyone else is scratching their heads over it. I set up a Backup MX on SM17x (Build 6956). I had put a few single IPs in there along with a few IP ranges. When I tested everything, I kept getting bounces stating "No Such User Here". I scratched my head because I have set up literally dozens of Backup MX servers on SM with no issues. I double and triple checked my work, checked my DNS, checked logs. Found nothing of interest. Was boggled. Called ST. Initially spoke with Ben. SM 17x on this server was like 17 or 18 versions behind. Ben advised an upgrade. I upgraded to the current build (7188). Tested again. Same bad results. Called ST again. Spoke with Rod. Tested a few things. Decided to break the IP range down into separate IPs. That fixed the problem! So I believe I found a bug. Again. Rod is aware of this and I'm sure they will fix it in a future build. I hope this helps save someone a lot of time :)