Here are some questions to help you understand your network requirements:
Do you want to obfuscate internal network information?
Do you want to provide redundancy of network services?
Do you want to limit available data on access layer hosts?
Do you want to simplify end-user settings, for example, have end users enter a single mail host that does not have to change if you move them?
Do you want to reduce network HTTP traffic?
Answering yes to these questions suggests a two-tiered architecture.