An interview thank you letter sent after your job interview can be is an extremely effective and simple tool to help you stand out from the crowd.
Interview thank notes make a great impression, and very few people actually send one, so sending an interview thank you note is an easy way to show that you will go one step beyond the average person to do a great job.
If the employer has decided you're not a good fit for the position after the interview, sending an interview thank you letter will likely not change that decision.
On the other hand, if there's a close decision between you and another candidate, sending a thank you note after the interview can be just enough to give you the extra edge over your competition.
Since you never know how you compare with the competition at this stage of the hiring process, it's always wise to take this extra step and send a thank you letter.
The problem with sending a thank you note through the regular mail is that it will not arrive in time to influence the employer's hiring decision. The employer could be making a decision within the next day or two following the interview, so your interview thank you note must arrive the next business day. Sending it by email or hand delivering it (when appropriate) will ensure that this happens.
Sending an interview thank you letter after a job interview is an extremely simple and effective way to add to the employer's interest in you as a potential employee. It's amazing to me that so few people bother to use this strategy. I've heard plenty of stories from employers who have been positively influenced by a well written, sincere interview thank you note, there's really no good reason to fail to use this proven job search strategy.