A multiplexor is necessary to achieve horizontal scalability (the ability to support more users by adding more machines), because it provides a single domain name that can be used to connect indirectly to multiple mail stores. A multiplexor can also provide security benefits.
While MMP is managed separately from Messaging Server, the Messenger Express multiplexing is built-in to the HTTP service (mshttpd) that is included with the Message Store and Message Access installation.
Message stores on heavily used messaging servers can grow quite large. Spreading user mailboxes and user connections across multiple servers can therefore improve capacity and performance. In addition, it may be more cost-effective to use several small server machines than one large, high-capacity, multiprocessor machine.
If the size of your mail-server installation requires the use of multiple message stores, your organization can benefit in several ways from using the multiplexor. The indirect connection between users and their message stores, coupled with the ease of reconfiguration of user accounts among messaging servers allows for the following benefits:
Simplified User Management
Because all users connect to one server (or more, if you have separate Multiplexor machines for POP, IMAP, SMTP or web access), you can preconfigure email clients and distribute uniform login information to all users. This simplifies your administrative tasks and reduces the possibility of distributing erroneous login information.
For especially high-load situations, you can run multiple Multiplexor servers with identical configurations and manage connections to them by DNS round robin or by using a load-balancing system.
Because the Multiplexors use information stored in the LDAP directory to locate each user’s Messaging Server, moving a user to a new server is simple for the system administrator and transparent to the user. The administrator can move a user’s mailbox from one Messaging Server to another, and then update the user’s entry in the LDAP directory. The user’s mail address, mailbox access, and other client preferences need not change.
If a message store grows prohibitively large for a single machine, you can balance the load by moving some of the message store to another machine.
You can assign different classes of users to different machines. For example, you can choose to locate premium users on a larger and more powerful machine.
The Multiplexors perform some buffering so that slow client connections (through a modem, for example) do not slow down the Messaging Server.
Decreased Cost. Because you can efficiently manage multiple Messaging Servers with a Multiplexor, you might be able to decrease overall costs by purchasing several small server machines that together cost less than one very large machine.
Better Scalability. With the Multiplexors, your configuration can expand easily. You can incrementally add machines as your performance or storage-capacity needs grow, without replacing your existing investment.
Minimum User Downtime. Using the Multiplexors to spread a large user base over many small store machines isolates user downtime. When an individual server fails, only its users are affected.
Increased Security. You can use the server machine on which the Multiplexor is installed as a firewall machine. By routing all client connections through this machine, you can restrict access to the internal message store machines by outside computers. The Multiplexors support both unencrypted and encrypted communications with clients.