I added Boomerang based on your recommendation. I already use Mailbox for iPhone to schedule emails to reappear at some given time. I've used Boomerang primarily to delay the sending of an e-mail ("Send Later") and to schedule emails to reappear in my inbox if I don't get a reply. I find "Send Later" particularly useful, since it removes the rationalization that "I'll compose this e-mail later, since I can't send it out until later".