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)
Whether you're responding to an interview request by letter using email or snail mail, always express your appreciation for the opportunity to move forward in the selection process. The first sentence of your reply should say how pleased you are for the invitation to interview with the recruiter or the hiring manager. In fact, re-state the interviewer's name if you receive a letter from a recruiter who is merely scheduling the interview for someone else.