Start sentences with the @ button next to the 'Reply' box to assign to-do items to yourself or others.

Choose someone you want to mention and write a task after that, for example, "@charlie, remember to buy some stamps". After it's sent, you can swipe left and check the "Mentions" list to manage all the tasks mentioned in this email. You can also click the "All mentions" on the same page to review all the tasks within your inbox.