Users rely on their applications, from word processing software to e-mail clients such as Outlook, and these applications and their data have to be available to be useful, else productivity will suffer. Outlook and OWA do not just rely on DNS and Exchange to work. Without a properly configured and healthy TCP/IP network, nothing will work. Yes, users can "work" in offline mode, but for how long?

Outlook and OWA must be able to process Exchange data, and that data must be available to users. When considering Exchange availability, we have to evaluate the network design in order to locate any potential single points of failure or bottlenecks that may interrupt the messaging service. If any are found, redundancy should be added or changes to the network topology should be made to mitigate these.

Consider a network connection, for example, between two Exchange servers separated by a switch. Normally, they would be connected through a single link to the switch...