Possible Duplicate:
Manage un-replied GMail conversations
Is there a way to get a notice if I send an email to someone expecting a reply, but after a few days they haven't replied to it?
Either something already in gmail, or an extension would also work.
For example if I send a friend a message like "hey I'm coming to New York next weekend, where should we meet" I would like to be notified if they didn't reply to it so I can remember to try to reach them through other means.