Unique Id for Tracking Emails

We would like to add a unique id to the form subject line and possibly in the message. This id will be used for internal tracking to determine where the email came from and used as a reference. It must be different for each email but contain a constant prefix.

I don’t see any way to do this from the interface. Is this possible?

thanks,

http://wordpress.org/extend/plugins/si-contact-form/