SaneRemindMe notifies you when an email you sent was not answered by a certain time. Just send the email as usual, and CC or BCC 1week@sanebox.com (or 5min@sanebox.com, or Jan1@sanebox.com - pretty much any combination of dates and times works, but you can see detailed instructions here.)
SaneBox will track your email and put it back at the top of your Inbox at the time indicated, unless the email has been replied to. We will also create a folder called SaneRemindMe, which will hold all of your emails awaiting reminders.