I think another flying creature that would be useful in bringing mail from magic folk to magic folk would be a falcon, because their grip is pretty tight. Having this tight grip means that they will not lose the mail. However, someone would have to train it to not grip the mail to tight. Although a trained falconer can do this very easily. This would also bring about a new job to the magical world.
(This is a essay by me. I'm at work right now and can't save it any other place. Sorry I will delete this as soon as I get home.)