In my culture I was born in, it is better to avoid abrupt sense when we write something new. Asking a question could be abrupt, and we typically add one subordinate clause which mentions like "I am sorry to ask, but..." in my native language. I tend to apply the same rule when I ask something in an e-mail in English. Typically, I add "One question." before writing an question itself in an e-mail. Does this sound unusual in a formal e-mail?
Also, I use "One question." as "I have one question.". Is it better to write out to avoid confusion or misunderstanding which I do not recognize?