But is a word, I like as it helps me in my conversation to disagree with a statement or discussion. Like 'But', the other word is, 'However'.
Teacher to student
You said you had prepared very well for the English exam but your answer paper tells a different story.
You say you don't believe in God but I've seen you at the Ganesha temple, next to the office many times.
Mr.X, a nice monthly sales presentation but, in my view, the figures mentioned are exaggerated as the sales bill book show last month's sales to be below average.
But and However are words that can be used in formal communication and in professional places without being very rude to the other person.