I mailed out a letter to her June 16, 2021 and received my autograph today, July 19, 2021.
It does look like an authentic signature even though she didn’t address it to anyone. (You can hold it up to the light to see if the signature matches the gloss of the picture. If it matches, it’s pre-print, if you can see the difference in marker, then it’s authentic).
When I write to a celebrity (typed letter), I usually include my return address on the top. Whoever is in charge of Glenn Closes mail, cut out the return address part of my letter and taped it on the envelope.
I thought it was funny.
Here’s the pic of the autograph:

Another funny story, is that I wrote to the actor who voiced Gaston from the animated version of Beauty and the Beast, he signed my letter and mailed it back to me. Oy.