Wow! Sometimes authors will comment in their email when they return an article that they appreciate the editing and think I did a good job, which is nice (and it immediately gets labelled "Kudos" and saved). But the authors I worked with a couple of weeks ago (the ones who described themselves as "meticulous" and wanted to talk to me on the phone) sent an actual snail-mail letter to the managing editor praising the editing on their article, and called me out by name. The managing editor emailed it to all the senior management and me with the subject line "More proof of greatness!".
I may have cried a little bit. That totally made my day.