Fun fact, a postcode can represent up to 100 addresses, apartment buildings with more than 100 units have multiple postcodes. Sometimes third-party systems will only recognize a subset of those postcodes (e.g: a system might assume a building only has 1 postcode) which can lead to some residents being in the geographic region for a service while others are not despite being in the same building.

Also postcode data is licensed by the Royal Mail, not free.