And of course I don't know the details, but I don't think the fact that she didn't reply means she didn't truly appreciate your heartfelt letter. I have never heard that it is expected that someone offer thanks for a thank-you note. She may have felt very flattered and have kept your words close to her heart. Maybe she felt it best to not reply. Many if not most DE recipients wouldn't want to be in too close contact with the donors and she could have felt it was the right thing to do to have your letter be the friendly, warm closure to the relationship.