11.1 Introduction

When describing the key features of Exchange 2000, and Windows 2000 for that matter, Microsoft said that they had focused on the "abilities." Examples given were scalability, reliability, manageability, and flexibility. One of the key "abilities" of Exchange 2000 is the ability to scale the product not only vertically (bigger boxes with more users), but also the ability to scale the product horizontally through the incorporation of functional partitioning. In short, this means that specific functions, such as message transport or HTTP access, can be deployed on servers dedicated to a specific task or function. A common term for this is separation of services, and it is usually realized in the form of front- and back-end servers. Basically, a front-end server accepts requests from clients and proxies those requests to the appropriate back-end server. In the case of SMTP, the front-end SMTP server would deliver a message to the appropriate back-end server that hosts the recipient's mailbox. This delivery is accomplished through normal SMTP transport and not via the type of proxying that takes place with the other Internet protocols. With SMTP, both the front- and back-end servers would provide full SMTP services, including store and forward.

While various services can be hosted on a front-end server with Exchange 2000 namely, HTTP, NNTP, POP3, IMAP4, and SMTP the sections in this chapter focus on SMTP.

11.2 Front- and back-end servers

The architecture for Exchange 2000 allows for the implementation and configuration of front- and back-end servers. The specific architectural change...