Whilst mammals that exist today cannot tolerate a body temperature much in excess of 40 degree C this is the result of climatic conditions and the evolutionary pressures that have shaped evolution on Earth. With different climatic conditions and different evolutionary pressures some organisms might well have evolved to survive at much higher temperatures.
There is nothing in the basic definition of a mammal that would rule out a high temperature existence. However such a creature would be very different from the mammals that we know. Most existing mammalian biochemistry would have to be modified or replaced as the enzymes (and many other molecules) used only operate at relatively low temperatures and are denatured at higher temperatures.
But temperature tolerant enzymes do exist as can be seen from thermophilic organisms so there is no absolute restriction on the basis of biochemistry. So if the evolutionary selection pressure were sufficiently great any number of exotic high temperature enzymes might evolve.
How high the temperature might get is impossible to know or calculate as there are too many variables and complexities, however there must be an upper limit to what is possible with chemistry. Some archaea can survive temperature in excess of 120 degrees C, but the limit for higher more complex life forms must surely be considerably less than this.