Consider a scenario in which Smaug was not killed in T.A. 2942, continuing to live to T.A. 3019, the year of the Battle for Middle-earth. Now let's say that he and Sauron had a major disagreement concerning the sharing of the treasure within the Lonely Mountain, at which Smaug grew so angry at the thought of losing even just one piece of it, that he decided to teach Sauron a lesson by attacking Barad-dûr.
Smaug had easily destroyed the city of Dale and Lake-Town, and he easily fire-blasted and smashed his way into the Lonely Mountain so I wonder if it would have been much of a challenge for him to destroy Barad-dûr despite its large size. I doubt that Barad-dûr would have had anything in its weapon arsenal that would have penetrated Smaug's scales and he would likely have had no trouble with taking out the nine Nazgûl riding on their fell Beasts.
If he had wanted to, could Smaug have destroyed Barad-dûr?