Thanks for reaching out with your suggestion! This functionality is actually already implemented within SmarterMail, though I feel it could use some improvements. To start a task from an email, click on Add > Task in the toolbar. You'll get a task window that looks like this:
Once you configure and save your details, a new tab will appear on the email like this:
I think adding a sort/filter to tasks so you can easily find ones that are related to emails would be a good addition to this functionality, as well as adding a direct link to the related email from the task itself. Play around with it a bit when you have a chance...What other improvements would you like to see?