The “Flag” capability in Outlook can really help here. For any email that has a question in it, simply select the Tags pull-down, and select an appropriate flag (in our case, Next Week).
What about your iPhone you ask? Well, unfortunately, this is a 2-step process. Newly created emails in the iOS email app do not allow you to “flag” them. Instead, you first need to navigate to the Sent email folder on your iOS device. Once there, you can then flag it.