Microsoft IIS Web Server and ASP.NET

I'd like to implement domainkeys/dkim and also alter the SMTP "MAIL FROM " to include a database id so I can quickly link bounced mail to the subscriber. IIS doesn't seem to have the ability to do this.

Are there any other inexpensive solutions?

You have a few options come to my mind.

• Send mail though a full blown email server which can track which emails bounce. And which you can process the log files easier to get to the details you need.
  
• Is to write your own SMTP mail send service which better links to your software to track the error messages.