There is an email address to send errors and such. A secretary prints them and he marks up the hard copy (if necessary) which is returned with a reward check (when appropriate).

I have received one actual email from him, when I pointed out an error in the balances at the Bank of San Serriffe (https://www-cs-faculty.stanford.edu/~knuth/boss.html).

The email looks like it was typed by him - the style is right and it is signed "Don" - but the From: address has someone else's name.