IIS may stop responding (hang) or may crash when you try to close it if an SMTP transport event sink that handles the IMailTransportOnPostCategorize::OnMessagePostCategorize event is registered with the SMTP service

The Microsoft Internet Information Services (IIS) service may stop responding (hang) or may crash when you try to close it if a Simple Mail Transfer Protocol (SMTP) transport event sink that handles the IMailTransportOnPostCategorize::OnMessagePostCategorize event is registered with the SMTP service.

This problem may occur if the SMTP transport event sink is still processing asynchronous events at the same time that you try to close the IIS service. When you close the IIS service, it releases memory that the SMTP transport event sink needs in order to process asynchronous events.

To work around this problem, use one of the following methods:

• Disable the SMTP transport event sink before you close the IIS service. When you do this, make sure that the SMTP transport event sink has finished processing any asynchronous events.
  
  Note If you are not sure about how to determine whether the SMTP transport event sink has finished processing an asynchronous event, contact the manufacturer of the event sink.
• Use the iisreset.exe command to stop or restart the IIS service.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

For more information about the IMailTransportOnPostCategorize::OnMessagePostCategorize event, visit the following Microsoft Developer Network (MSDN) Web site: