The landlady of a block of flats in Cologne discovered the trove of mail in the basement after a water pipe burst on Friday.
Police were alerted and arrived on the scene to find thousands of undelivered letters piled into around 100 postal boxes.
Most of the letters were addressed to recipients in Mainz, which is about a two-hour drive from Cologne.
Police are investigating a 27-year-old man who had worked as a postman until June in Mainz and then moved to Cologne.
The letters will now be gradually sent out to their rightful recipients.
A postwoman near Dortmund was sentenced to two and a half years in jail last year for throwing away 800 letters.
And in Spain last month, an ex-postman was arrested for stealing thousands of letters over the course of ten years.