Reading Perilous and Fair: Women in the Works and Life of J. R. R. Tolkien, eds. Janet Brennan Croft and Leslie A. Donovan, I came across this fascinating snippet in John D. Rateliffe's essay, "The Missing Women: J. R. R. Tolkien’s Lifelong Support for Women’s Higher Education":
The exact quote is that Renault's card was "perhaps the piece of 'Fan-mail' that gives me most pleasure". Considering how much fan mail he must have had, that is a very strong statement indeed.