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MAPI problems when Exchange Server and Microsoft Outlook share

There are good reasons why Outlook and Exchange should not be installed on the same computer, as this tip explains.

Microsoft does not support installing any version of Outlook on the same computer as any version of Exchange Server.

On the face of it, this may seem absurd and unfair, but there are reasons for this that have to do with different versions of the components both programs use.

Both Outlook and Exchange Server use a standard Microsoft programming interface called MAPI (short for Mail Application Programming Interface). MAPI is used by Outlook and Exchange for radically different things. More specifically, the way MAPI is implemented for the two systems is radically different.

Exchange's implementation of MAPI needs to scale well to hundreds or thousands of users, and has been written and debugged from that point of view. Outlook, on the other hand, was written from the point of view of a single user, and so many of the functionalities that are most robustly developed in Outlook's MAPI (RPC over HTTP, for instance) are of that variety.

Outlook implements MAPI through a component called the "MAPI stub." The stub is essentially a redirector, used to "farm out" the various MAPI calls to the different ways MAPI may be implemented on the same computer. (This was done so that multiple mail clients could coexist on the same computer without eating each other alive, as it were.)

Exchange, however, does not use the stub library, since that would cause a performance penalty not acceptable in a server environment. As a result, Exchange's functions have to be satisfied directly by Exchange's implementation of MAPI, and Outlook has to work through the MAPI stub. If both Exchange and Outlook are installed on the same computer, the two methodologies may wind up interfering with or overwriting each other.

If there is a real and urgent need for a mail client of some kind on a server running Exchange, there is always Outlook Express (even though it is very primitive), which does not use the full implementation of MAPI calls and is already present by default in Windows Server 2000/2003. Another possibility is a non-Microsoft mail client (Thunderbird, Eudora), or running a full copy of Outlook in an emulated machine using VMWare or Microsoft Virtual PC.

(Thanks to Stephen Griffin of Microsoft, whose blog was the source for this tip.)


Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.


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