INFO: Suitable Applications for Exchange Server Event Scripting

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Because the Microsoft Exchange Server 5.5 Scripting Agent permits scripts to run against individual mailboxes, many developers have attempted to write Inbox Agents in VBScript. These attempts have yielded poor results, because the Event Scripting Agent is not designed to replace traditional mailbox agents written in C/C++. Such agents are normally used to scan every piece of mail in or out of the system for viruses, or to archive every message for government record-keeping purposes.

There are several vital limitations to the Microsoft Exchange Event Service that render it unsuitable for such applications:

• There is no guarantee that a script will see every message. The Event Service fires events asynchronously with respect to the Microsoft Exchange information store. This means that the information store does not block an event script (wait for it to run) before letting other processes or users access the folder or message. Consequently, that message could be moved, copied, deleted, or modified before a script has a chance to run. For example, a script to process all outgoing messages, applied to the OnMessageCreated event in the Outbox folder, may not react to the event before the new message has been delivered and deleted from your Outbox.
• Some of these types of scripting applications may face problems of scale. Depending on the application's purpose, either the script would have to be manually installed in every folder for every user, or event volume would simply be too high to process with an interpreted language like VBScript.
• The Event Service is single-threaded. No matter how many scripts are registered, only one can run at a time. The performance consequences are substantial. For example, a script that requires two seconds to execute, only allows one script to execute every two seconds, for a maximum of 30 scripts per minute. At that rate, 600 scripts would require 20 minutes to process.
• Event Scripts run in a privileged Windows NT security context, that of the Exchange Server service account. One grave consequence of this arrangement is the possibility that a script can open any mailbox on the server and make changes to the contents, without the owner's knowledge. The Exchange 5.5 Release Notes state: "The Scripting Agent is not a safe, general purpose replacement for the Inbox Assistant rules because it allows new code to be installed and run on your servers."