Thank you letters following interviews are a professional courtesy. Immediately after you have an interview, send a brief thank you to the potential employer for the interview. Many employers expect to receive thank you letters within a couple of days of the interview. Thank you letters not only demonstrate good manners, but also provide another opportunity to present a positive image of you. Note that a thank you letter should always be sent, even if you do not expect to receive a job offer from this employer or if you have decided that you are no longer interested in the position for which you interviewed.
Your reply to your new employer should be brief, personalized, and written in such a way as to reinforce the employer's decision to hire you. Be sure to indicate the date on which you will start work so there will be no misunderstanding.Refer to details of agreement, e.g., job title, responsibilities, salary, starting date, etc.Express your appreciation and your pleasure in joining the staff.
It is very important to notify employers from whom you have received offers that you have reached a decision and accepted another employer's offer. State your appreciation for the time and interest that they have shown you.