iOS Sometimes, it's really important to know when an email you send gets read. MailTracker is an app that allows you to not only see when an email is read, but also where it's read and for how long.
MailTracker hooks into your Mail app so you don't need to worry about using another email app. Just send an email with the default Mail app, and MailTracker notifies you when it's read, where it's read, and for how long. You probably won't need MailTracker for every email you send, but it's helpful in a lot of situations where you need to know when an email gets read. It's free for up to two email accounts, and $US5/month after that.
MailTracker (Free) [iTunes App Store]