I also had a few such exchanges (mine would have been at the end of the 80s or early 90s, I think) -- by snail-mail in both directions. His replies came in enve

I also had a few such exchanges (mine would have been at the end of the 80s or early 90s, I think) -- by snail-mail in both directions. His replies came in envelopes addressed to me as "Dr. ..." or even once as "Prof. ...", which I found pleasantly amusing, as I hadn't actually graduated from anywhere, let alone reached such exalted heights. (His usual form of reply, as I recall it, would be a note pencilled on a copy of my letter.)