This morning, suddenly extension mobility service in my place goes inactive. When users access Services -> Extension Mobility, a message "Host Not Found" appears. There is connectivity issue between subscribers to publisher, but 30 minutes after the connectivity is resolved, the problem still occurs. It is resolved by restarting the publisher. I suspect there's something wrong with the EM service, unfortunately I haven't take any notice on it during the problem.
My question is, what can go wrong ? What system activities that can cause EM service unavailable. I'm quite sure there's no user interrupted activities that cause this.