Wrote to a composer a few years ago and pianist recently with pretty focused things I wanted to compliment, and they both clearly appreciated it because they sent very long, considerate responses (longer than what I had sent initially!)
Seriously it makes their day. They’re people too!
Some years ago I was trying to track down a comedy TV series I watched when I was a student, along with most other students at the time, which wasn't big enough to have been later issued on DVD. I emailed the agent of the actor/producer who starred in it and he gave me his mobile number and told me to give him a call. We talked for quite awhile about the history of the series and the fun they had making it, and he said if I couldn't find a copy anywhere (I eventually found it in a film archive) I could come round to his place and search through his garage because he had video tapes of it in there somewhere. He was really cool about the whole thing, and seemed happy that his work was still remembered 30-odd years later.
I had this experience some years ago writing to an author, too. Long reply back.
I have corresponded at length with composers who seemed to appreciate the interest. On the other hand, I know a composer (not even that famous, just successful enough) who has his copyist answer all correspondence with stock replies, since fans don't do anything for him and just annoy him; they aren't part of the ecosystem of commissioning works that is how he gets paid.