Many companies don’t like to share employee names or email addresses with candidates. In that case, sending the thank-you note to HR and asking them to forward it is totally fine. That works just as well, IMO.
Hmm. I haven’t seen physical mail at my firm in a long time. It isn’t part of how we operate anymore. We don’t have interoffice mail, and I haven’t received any outside mail in years.
If I did receive something in the mail from a candidate (whether handwritten or not), I wouldn’t see it as “thoughtful” - I’d see it as “weird” or out of touch. It would honestly make me question whether they understand how modern companies operate. I think it would be a negative for me.