purchase third-party software or write an event sink yourself. My personal preference would be to code the solution instead of purchasing third-party software.
I know you said you are not a programmer, but Microsoft has some pretty good source code that demonstrates how to do this. Have a look at How to use SMTP transport events to add a disclaimer to outgoing SMTP messages, Add a disclaimer to outgoing SMTP messages in Visual Basic script and Add a disclaimer to outgoing SMTP messages in Visual Basic for sample code that shows exactly how to do this.
And there may be more here.
This was first published in January 2003