The Best E-Mail I Have Ever Received:
Received recently from a former student of mine, with identifying information deleted:
Professor Kerr,

I don't know if you remember me, but my name is []. I am a member of the Class of [] and was a student in your Criminal Procedure and Computer Crime classes.

I write this simply to say "Thank You." Your passion for the subjects you teach definitely came through, and it helped instill in me the desire to work in the field of criminal law. I recently received an opportunity to work as an ADA at the [] District Attorney's Office. After attempting a variety of other professions, it feels great to know that I finally found something I am truly passionate about, and I'm not sure that would have been possible without enjoying the subjects you taught. So while the generalization is that professors are unappreciated, please know there is at least one student who is grateful for your work.

All the best,
[Former Student]
For a professor, it just doesn't get any better than that.