From the Transmission Repair Guy:
The ideal fluid temperature is under 175 degrees, but as fluid ages it starts to break down and lose its capacity to cool down the transmission. This is when transmission overheating occurs. At 220 degrees, varnish forms. At 240 degrees, seals start to harden. At 260 degrees, you’ll often experience transmission slipping as plates slip. At 295 degrees, seals and clutches start to burn and fluid forms carbon, at which point transmission failure might happen soon. But with every 20 degree drop in fluid temperature, your transmission’s lifespan can approximately double! That’s a pretty significant difference, a decrease from 220 degrees to 195 degrees can allow your transmission to last another 50,000 miles!