Remailing means that Deutsche Post can assert payment claims for redirected domestic mail against a domestic sender. This involves the use of the accounting system for international mail by domestic senders. These send their advertising letters addressed to domestic recipients to Germany via a foreign postal company (e.g. the British Post).
The foreign postal company receives the full postage revenue even though it only sends the letters unprocessed to Germany. Deutsche Post does the laborious job of sorting and delivering, for which it only receives a small fee. It therefore makes use of the statutory option for extensive mail items to demand the difference between the domestic sender and the domestic tariff.