XFOR: How to Limit Recipients on Outbound SMTP Messages (196818)

SUMMARY

The number of recipients specified in the To field of an SMTP message can not be limited by Microsoft Exchange Server 5.5. However, the manner in which the Exchange Server Internet Mail Service processes an SMTP message can be altered by specifying a maximum number of recipients in a registry entry. The registry entry setting affects only the outbound mail.

After you implement the registry change, a message exceeding the maximum number of recipients will be sent by the Internet Mail Service to an SMTP host in batches. Each batch will include a number of recipients equal to or less than the maximum number of recipients allowed. See the following More Information section for details.

MORE INFORMATION

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys And Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).

To modify how the Internet Mail Service processes messages based on the number of recipients, follow these steps:

On the Microsoft Exchange Server computer, start Regedt32.exe.

Find the following registry key:

   HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/MSExchangeIMC/ 
   Parameters

NOTE: The above is one path; it has been wrapped for readability.

On the Edit/Add value menu, add the following:

   Value Name: MaxRecipients
   Data Type: REG_DWORD
   Data Value: Enter the maximum number of recipients that will be allowed
   on a single transmission of an SMTP message.

Exit Regedt32, and restart the Internet Mail Service.

The MaxRecipients registry entry will affect the Internet Mail Service's behavior in the following way:

Suppose the MaxRecipients hex value is set to 0xFA. This translates to a decimal value of 250, which is the maximum number of recipients that the Internet Mail Service will process in an initial transmission of a message which contains more than 250 recipients in the To field.
If a message is addressed to 500 users, the Internet Mail Service will process the first 250 recipients in the RCPT TO commands.
It will then transmit the message data in a DATA command.
After the data is finished, the Internet Mail Service will process the final 250 recipients in the RCPT TO commands.
The message data is sent again in the DATA command.

NOTE: Using this registry entry may have an adverse effect on the performance of high volume Exchange Server Internet Mail Service computers because a message may be transmitted multiple times. Typically, this registry entry is most useful when you need to work around the limitations set on the SMTP host to which the Internet Mail Service is sending mail.