More back-story and a picture of the envelope can be seen at The Sneeze.

Source
Transcript
CHARLES M. SCHULZ
NUMBER ONE SNOOPY PLACE
SANTA ROSA, CALIF. 98401
December 23, 1986
Dear Randee:
I can't believe that you are so ready to leap in with your criticism.
Fifteen years ago, when the strip was drawn, I believe there was only one black in the National Hockey League. Does pointing out this fact make the strip racist?
Best regards,
(Signed)
Charles M. Schulz
CMS:jm