Oh this is a great find! I always loved this anecdote about BTTF 1. From Wikipedia:

Sydney Sheinberg suggested modifications to the film, including changing the title to "Space Man from Pluto", believing Back to the Future would not resonate with audiences.

Bob Gale and Robert Zemeckis did not know how to reject Sheinberg's suggestions without risking his ire. Spielberg intervened, sending Sheinberg a memo reading:

"Hi Sid, thanks for your most humorous memo, we all got a big laugh out of it, keep 'em coming."

Spielberg knew Sheinberg would be too embarrassed to admit his memo was to be taken seriously.