How to limit the number of recipients a user can address a message to.

Locate the following key in the registry:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\
ParametersSystem


On the Edit menu, click Add Value, and then add the following
registry value:

Value Name: Max Recipients on Submit
Data Type: REG_DWORD
String: A value equal to the max. recipients you
wish to allow
Quit Registry Editor.

This limit also applies to messages transferred into the store by the
Internet Mail Service. An event 4117 is logged and the message is moved to the Archive folder.   The message is not delivered and it does not send a non-delivery report. 

If a user attempts to send a message to more recipients than is designated by the Max Recipients on Submit parameter and the user has the new Emsmdb(32).dll, the following error message may be displayed when the user attempts to send the message:
The item could not be sent. The number of recipients on this message
exceeds the upper limit configured by the administrator.

If a user attempts to send a message to more recipients than is designated by the Max Recipients on Submit parameter and the user DOES NOT have the new Emsmdb(32).dll, the following error message may be displayed when the user attempts to send the message:
The item could not be sent. The client operation failed.

Note the following when you add the above registry key:
* You do not have to stop and restart services for this change to take
effect.

* The number you choose applies to total recipients associated with a
message (To+Cc+Bcc=Total).

* You cannot circumvent the limit by using a personal distribution
list; it is evaluated for number of entries before the send.

* Server-based distribution lists allow you to exceed the total number
of recipients if you have proper permissions.

* The new Emsmdb32.dll file referenced in this article is included in
Exchange Server 4.0 SP2 and all later versions of Exchange Server.