How to set a limit on the number of SMTP messages in queues

If you want to limit the number of Simple Mail Transfer Protocol (SMTP) messages that may be queued on a server that is running Microsoft Windows Server 2003 or Microsoft Windows 2000 SMTP Server, you can use the MaxMessageObjects registry key. If the limit is reached, no additional incoming messages are accepted by SMTP.

If Microsoft Exchange is not installed on the computer, you can install the SMTP protocol in Internet Information Services (IIS).

Note By default, the SMTP service is installed with Windows Server 2003 and Windows 2000.

To install the SMTP protocol in IIS, follow these steps.

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

1. Click Start, and then click Control Panel.
2. Click Add or Remove Programs, and then click Add/Remove Windows Components.
3. Click Internet Information Services (IIS), and then click Details.
4. Click to select the SMTP Service check box, and then click OK.
5. Click Next, and then click Finish.

To set a limit on the number of SMTP messages that can be queued, follow these steps:

1. Click Start, click Run, type regedit, and then click OK.
2. Expand HKEY_LOCAL_MACHINE, expand Software, expand Microsoft, and then expand Exchange.
3. Right-click the Exchange key, and then click New Key.
4. Type MailMsg.
5. Right-click this new key, and then click New DWORD Value.
6. Right-click the new value, click Rename, and then type MaxMessageObjects.
7. Right-click the value, click Modify, and then set the value to the maximum number of messages that you want to be in queues at any time. The default (if the key is not present) is 100,000, which is the suggested limit.

The MaxMessageObjects key refers to the maximum number of mail messages that the system can allocate. When that number of messages is in a queue on the server, additional incoming messages that come in through the SMTP protocol are refused with a 4xx (temporary) error.

The actual number of mail messages that is allocated is 90 percent of MaxMessageObjects, up to a limit of 90,000.

To increase the number of objects greater than 90,000, set the MessageObjectsInboundCutoffCount registry key to the value that you want. Do not set the MessageObjectsInboundCutoffCount value for more than 90 percent of the MaxMessageObjects key because additional messages are created during message processing for DSN generation and content conversion.