I hope this email finds you. The email knows where it is at all times. It knows this because it knows where it isn't. By subtracting where it is from where it isn't, or where it isn't from where it is (whichever is greater), it obtains a difference, or deviation. The email delivery subsystem uses deviations to generate corrective commands to drive the email from a server where it is to a server where it isn't, and arriving at a server where it wasn't, it now is.
