You always want to wrap up any interview with a proper thank you letter. Sending it through an email is the new norm as hand written notes are outdated. Please make sure you get the interviewer’s business card with their email information or if it happened to go through a recruiter ask for them to share it with you.
My suggestion is to send the email the day after the interview. A couple of hours afterwards would also be acceptable but only if you had an interview earlier in the day. I like to tell candidates to wait because it shows you have been more thoughtful about the email when you send it out. It also reminds them the next day that they did meet with you and that they should follow up with regards to your candidacy.
I have had candidates who have sent letters to hiring managers that just consisted of “If you have any questions let me know. Thank you.” Not addressing who it is and not signing their name and in worst cases sending it through an iPhone which says it on the signature. Come on, do you really think that shows you are interested in the role? My other suggestion would be to send it through an email that is more professional than “PrettyBabyKitten@email.com”
Here is a basic template that I would suggest to use. It will get the message across that you are interested in the opportunity.
Thank you for taking the time to meet with me. It was very exciting to have the opportunity to meet with your firm. I got a great sense of who you are as a company and feel strongly that my background would be a great fit for this role. I look forward to potentially working together in the future.
Feel free to add in the body of the email a couple of key points that you felt you found particularly fascinating or interesting about the role or refer to something positive that happened in the meeting. It will make your letter stand out more and feel personal. This would be particularly helpful if the interviewer is meeting with a couple of candidates at the same time.