Microsoft Mail is Microsoft's "legacy" mail system. Microsoft Mail is extremely limited in comparison to the power afforded by Exchange. Because of this it should not seem a suprise that Microsoft encourages customers to migrate and provides the tools necessary to make this process fairly smooth. Before jumping into how Exchange integrates with MSMail, it is important to have at least a basic understanding of how MSMail works.
MS Mail Concepts
Exchange is a client-server messaging system. This is contrasted with MS Mail which is a shared file-system messaging system. All email is stored in a shared location on the network, called a postoffice. Most processing occurs at the client side. When a user sends a message their mail client accesses the server and places the message directly in the recipients mailbox. The obvious downside to this type of system is that each user must have the ability to write directly to each other users mailbox. In addition, because the server is essentially "stupid" it does not inform the client that mail is waiting. Rather, the client must continually poll the server to check for new mail. This also has obvious drawbacks in that it creates an inefficient use of network bandwidth.
MS Mail systems may have more than one post office. Directory information is exchanged between post offices via the MS Mail Directory Synchronization Protocol. In such an environment, one server is designated as a Dirsync Server. It is the Dirsync Server that maintains the primary, or master, copy of the directory. The other servers that are participating in directory synchronization are called Dirsync Requestors.
The actual process of directory synchronization occurs during three distinct, timed events. Be aware of these events for the exam:
Dirsync Requestors send the address list for their postoffice to the Dirsync Server
The Dirsync Server combines all entries from received from the Requestors into a Global Address List. It is also during this time interval that the Global Address List is sent back out to the Requestors.
The Dirsync Requestor rebuilds its Global Address List.
The MS Mail Connector for Exchange
Now that the basic concepts of MSMail have been covered the following sections will look at how this all relates to Exchange integration. Exchange provides the MS Mail Connector for the integration between the two mail systems. The MS Mail Connector is comprised of the MS Mail Connector Interchange, the MS Mail Connector Post Office, and the MS Mail Connector MTA.
MS Mail Connector Interchange: This is an NT service that basically provides for transformation between the differing message formats. It reads messages ENTERING the Shadow Post Office and if destined for an Exchange recipient, translates the message and places it in the Information Store. Likewise it puts messages from Exchange users headed toward MS Mail users and places them into the Shadow Post Office.
MS Mail Connector Post Office: Exchange implements an MS Mail Post Office referred to as a Shadow Post Office. It is much like an ordinary MS Mail Post Office, except it is used only TEMPORARILY. As messages come into the connector from another MS Mail system they are temporary written to the Shadow Post Office before being translated (by the Interchange). And before messages go OUT from Exchange to an MS Mail system they are first translated and placed into the Shadow Post Office ... where they will be then sent off to the corresponding MS Mail system.
MS Mail Connector MTA: This is the unit responsible for the actual transfer of messages between Exchange and MS Mail. Remember that MS Mail system servers are relatively stupid. They require a third part to transfer messages between post offices. On the Exchange side, this is it. There can be up to 10 instances of the MTA running for an MS Mail Connector (each server can only have 1 MS Mail Connector installed). This allows a single connector to interface with 10 different Post Offices.
Directory Synchronization Between Exchange and MS Mail
Exchange and MS Mail can share directory information through the Directory Synchronizaton Agent (DXA). The DXA is the Exchange component that provides support for the MS Mail Directory Synchronization Protocl, mentioned previously. By implementing the DXA, Exchange users will be able to see MS Mail recipients as custom addresses in the GAL and likewise MS Mail users will be able to see Exchange users.
The DXA allows Exchange to act as either a Dirsync Server, or a Dirsync Requestor. It can only play one of these roles and there can not be more than one Dirsync Server per site.
Once implemented, the DXA operates much like any other MS Mail server, participating in the three timed events mentioned earlier. A note for exam purposes, you can force a T2 event(compiling the GAL and sending to requestors) when the DXA is operating as a DirSync Server by STOPPING and RESTARTING the Directory Synchronization Service (Control Panel).
Also note that you can control which Exchange recipients are included in the dirsync process through TRUST LEVELS of 1-100. Users with a trust level GREATER than that established by the connector will NOT be replicated. In other words, John has a trust level of 50 and you configure the DXA with a trust level of 20, John will NOT be replicated.