If you take a moment to think about it, we measure out 14grams of precisely ground, expertly roasted coffee beans that have been grown and cared for by farmers and processed based on sometimes generations of experience and knowledge.
We then tamp the grinds to a predetermined pressure, loaded into a precisely engineered portafilter basket that has been pre-heated on the espresso machine, under an exacting 15 bars of water pressure, heated to the perfect temperature of 91 degrees, resulting in 60 mls of deliciously smooth espresso.
So while the extracted coffee has a warm and cosy path through the group head, into the grinds and out the the preheated spout of the poetafilter, if you haven’t preheated your cup, the temperature of the extracted coffee is going to be different once it reaches your lips. That in itself is not such a bad thing, some people enjoy their coffee a little coole, and that’s perfectly fine, however the problem is the coffee liquid exposed to the cool cup drops in temperature faster that the coffee toward the centre of the brew. When these two mix, you end up with a cooler coffee that will affect the flavour, usually by reducing the sweetness.
I store my ceramic coffee cups on top of my espresso machine, but for the times that I forget, I’ll just rinse the cup with some warm or hot water and that will do pretty much the same job.