You can't do that. Well, actually you can, but it would take a ridiculous amount of effort - including developing a Windows service that is written in C and installing it on every one of the 5,000 client machines so that it can listen for signals from the Siebel server and use the Notes APIs to send email.
So I'll correct myself. You shouldn't do that.
But here's what you can do:
- Create a Mail-In Database on the Domino server
- Have code on the Siebel application server send an email to the Mail-In Database, using a pre-defined format to include the name of the user and whatever other data you need. You can use the Notes APIs for this, but really... don't bother. Just use SMTP.
- Create a background agent in the Mail-In Database and set the agent to run when new documents are created.
- Code the agent to read the contents of the mail that you sent to the Mail-In Database, parse out the name of the user and any other data that you included, and then send a message on behalf of that user.
Note that you only need to do this if there is data in Notes and Domino that needs to be included in the email that the Siebel server doesn't know about. If that's not the case, just take Andrew's recommendation and use SMTP on the Siebel server.