Giving visual feedback in emails for inactive task

System

We send out emails with url commands for almost all user and group tasks. I'm thinking that with the users we have here, they may get confused if they don't delete an email after they take action on it, or even worse, if it's for a group task and someone else has already done it. Anyone have any suggestions for limiting the confusion?

Aside from training, training, training, the best thought I've had so far is that I could put an image in the email, and set its src to a cgi that would check to see if the task were still active and display differently depending on whether it was or wasn't.