There was no "postal service" in the European Middle Ages. Letters were individually delivered by messengers. Frequently, the physical letter itself would only convey part of the message. The sender would tell the messenger the rest of it, to be delivered in person to the intended recipient.

The earliest organized mail service in Western Europe is under Maximilian I in the Holy Roman Empire at the turn of the 16th century, mostly for use by delegates to meetings of the Reichstag (imperial parliament).

This recent thread has some great information on medieval letters!