It's all a matter of style and consistency. Some choose to capitalize only the first word, e.g. (using your example) The title of this question. Others capitalize the key parts of speech in the title, excluding conjunctions, prepositions, and the like: The Title of T/this Question. In some cases, all the words in the title are capitalized:
How to Reply to Confirm an Interview. When confirming a job interview, type a one-page letter thanking the interviewer for the invitation to the job interview. Confirm the exact time, date and location of the interview to avoid arriving at the wrong location on the wrong day or time. Send the letter within 24 to 48 hours of receiving the invitation. (Creatas Images/Creatas/Getty Images)
Reply To An Interview Invitation. Hi! A company has sent me the following email to arrange a telephone interview with me. I've written the answer but I'd be really grateful if someone could check it. I find that my answer is too abrupt and I believe that what they want with this email is to check my English. I don't want to sound pompous either.