A hard-copy thank you letter should be written in the business letter format, while an email should be sent in the same format but without the heading (your return address, their address, and the date).
Thank you letter writing
The first paragraph should consist of thanking the interviewer for the interviewing you (remind him/her about the position you interviewed for and the date of your interview). You can also include information about your impressions about the company.
The second paragraph should state your interests in the company and include any additional information about yourself that was not brought up in the interview which would make you a good candidate for the position. You can also emphasize your qualifications that were already discussed during the interview (don't make this paragraph too long, try to keep it between 3-5 sentences, pick the traits that you think were most important to the interviewer and emphasize them).
The last paragraph should let the recruiter know that you expect to hear from them soon. Also let them know that you are available to come in again and are willing to discuss the job further. Write down your contact information again and what the best method and/or time to contact you is. To finish up the letter, thank them again for the interview.
Also see: Sample Interview Thank You Letter
Resources: Job Interview Tips
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