Reliability is a specific, but very important, performance metric. When a service is intended for daily work activities, and not just casual communications, an established set of performance metrics become important. These metrics need to be defined and monitored to ensure that the service is performing to client expectations, and that clients have clear expectations of the service. For example, one performance metric that clients will expect if the service is to be considered a daily work tool is an upper bound on the end to end delivery times of electronic mail within the supported infrastructure.