First of all, when you install Exchange Server, you have to plan all the roles this server is going to take. A role performs a very specific function within the environment. Usually only one of these functions is necessary, but we just have to think how many of each of these roles we want in our network, and where they are going to go. Now some roles can be combined with other roles, and some cannot be combined on the same server.
The server roles that we will discuss here are the mailbox server role, client access server, hub transport server, edge transport server and unified messaging server. The mailbox server role hosts mailboxes and public folders; so, this is where the actual database is. This server role supports clustering, which is very helpful for one to not lose their mailbox database. This type of server should always be protected behind a firewall and with a good antivirus. Do not forget to join the server in the Active Directory either, because this is a Microsoft requirement and uses fast accessible storage for mailbox databases. If you have many users and they send and receive a lot emails, you will need it for sure.
The client access server role allows the "non-mapi clients" to access the mailboxes. These mapi clients will actually connect to the mailbox server itself. Now when we talk about non-mapi clients we are dealing with POP or IMAP protocol, Outlook Anywhere, Outlook Web Access and finally ActiveSync. All of these are alternative ways for people to check their messages. This role must be placed in each Active Directory site, if this site has a mailbox server. Also, it must reside behind the firewall and, of course, it needs a fast, reliable connection to all the other mailbox servers.
The hub transport server contains a message categorizing and routing system that is used for delivering messages. It is a necessity to have a fast and reliable connection to the mailbox and global catalog servers. Moreover, it is required to place at least one hub transport server per site if that site has a mailbox server installed. You must also join all hub transport servers to the Active Directory domain.
The edge transport server role takes care of Internet message delivery, and is always deployed in an organization's perimeter network. It will accept all emails coming into the company. This is where anti-virus and anti-spam protection should be used. Here we apply the edge transport rules, used to manage the flow of email messages that users send or receive over the Internet. Using these rules, one can apply specific actions for any and all messages that meet specific criteria. For security purposes, this server role cannot be a member of the Active Directory.
Finally, the unified messaging server role allows us to integrate all of our emails, voice mails and facsimiles within Exchange Server. After installing this server, you can access everything just by using your Outlook mailbox. It is important for this server to have a continuous, uninterrupted connection with the global catalog server, mailbox servers and hub transport servers, since all of them are going to work together for delivery straight to your mailbox.
If you are also in need of routing telephone calls, do not forget to get yourself and IP-PBX or VoIP gateway device, to handle the routing.
Hope you got the basics of Exchange Server cleared up!