On a single Exchange server deployment, there is no 100% reliable way to accomplish this with an SMTP Transport Event Sink; even though KB273233 suggests that creating a second SMTP Virtual Server works. However, at startup the Exchange Information Store binds to the SMTP Virtual Server that starts first and you can not rely on the routing of the mail from SMTP VS 1 to SMTP VS 2 as the KB273233 proposes. Also note that under special circumstances the database can become corrupted if you use an SMTP Transport Event Sink to manipulate outgoing (MAPI) message contents. This is currently under investigation by Microsoft and a QFE to prevent the store corruption is under development. ****

There are 3rd party products that will do this too.