Starting with Exchange 4.0 SP1, an administrator can limit the number of recipients a mail message can have.
Use Regedt32 to navigate to:
On the Edit menu, Add Value name Max Recipients on Submit, as a type REG_DWORD. In the DWORD editor, using the Decimal Radix, set the limit.
NOTE: Max Recipients on Submit also applies to messages transferred by IMS (Internet Mail Service).
When the limit is exceeded, an Event Id 4117 is logged, the message is Archived, NOT delivered, and no NDR is generated.
If a user has the new Emsmdb32.dll file when they exceed the limit, they receive:
The item could not be sent. The number of recipients on this message exceeds the upper limit configured by the administrator.
If a user does not have the new Emsmdb32.dll file when they exceed the limit, they receive:
The item could not be sent. The client operation failed.
You do not have to Stop and Start any service for this change to take effect.
Max Recipients on Submit applies to the sum of To+Cc+Bcc.
Personal distributions list are evaluated, so they can not be used to circumvent the limit.
Server-based distribution lists (if you have permission) allow you to exceed the limit.
The NEW Emsmdb32.dll file is included with all versions of exchange from V4.0 SP2.