Check out the Inbox Zero methodology, which mentions the following step
Defer (Sometimes, you can't do things right now, create an appointment/task, delete the mail.)
although you could keep the mail labeled Deferred or in a Deferred folder, it's discouraged to do so as the information of your e-mail should in a note book and the only resulting data of the mail should be a task or an appointment. Don't keep the e-mail to reply to it later, as if you must do a task then it's no longer an e-mail conversation and you could as well write a new mail informing that you are done...
But, these are all generic guidelines which you don't have to follow that strictly. If you are a teacher, for example, you might need to keep e-mail submissions which you will correct by a certain date and send back those e-mails witch corrections. In which case having a Deferred -> Home Task X label or folder structure makes sense...