Effective 1st August, the Korean Post Office announced a postcode reform replacing the old 6-digit postal code structure ta a new set of 5-digit postal codes.
So far, Korea has established a dual postal code system in order to reduce the impact of these changes. However, it is highly recommended the the new 5-digit codes are used as soon as possible since the old postal code will expire on September 30th, 2016 (tomorrow).
After this time, only the new 5-digit postal codes can be used and any shipments with the old 6-digit codes will either fail or be misrouted, leaving to delayed deliveries.