First of all: E-mail is not the best way to ask people for commitment. Either ask people in person or on the phone. But e-mail is a great way to confirm what you’ve agreed on, send information and… to track delegations.
Personally I love using Culture Code‘s Things for tracking my own projects and tasks in a GettingThingsDone manner. I’ve composed a small AppleScript to use Things also for tracking any delegations or any other e-mail I absolutely need a response to.
Apple Mail unfortunately does not allow you to apply rules to e-mail that you send out, so you’ll need to BCC yourself or a dedicated e-mail address. Then just create a rule that applies to those incoming mails and execute the following Script. It will create a task in the “#Delegated” project I’ve added in Things and set the due date to 7 days from now.
Adjust the script to your liking and be more effective!