Here is a question I get at least once a week, “I interviewed with several people. Who do I send the thank you letter to?”
Generally speaking, send a thank you email to every person with whom you interviewed. If you really want to stand out, take the time to personalize each email with something specific that that specific interviewer brought to the table.
For example, you might add in something like this: “I appreciated learning more about how your sales team partners with IT to develop proposals and presentations. It gave me an even clearer idea into the kind of collaboration I could expect at XXX and made me even more enthusiastic about the possibility of joining your team.”
In this job market, you need to seize every opportunity to reassert your interest and differentiate yourself. A thoughtful and personalized post interview thank you letter is a great chance to do that.