High mileage vehicles may develop a leak from one or more of the transmission oil seals.Google+
Car Problem Reports
Mercedes-Benz E320 4MATIC Transmission Oil Seals May Leak on High Mileage Vehicles
Mercedes-Benz E320 4MATIC Problem
Visitor, 2003 Mercedes-Benz E320 4MATIC, 111,689 mi
Transmission fluid leaks when at highway speed for >5min. Get some blueish smoke from it burning. Possible overfill of transmission fluid? The car is new to me.