"Thank you" letters

should be written promptly and sincerely. 
We were invited out recently and, the following day, we sent a thank you card because our hosts had made a very special effort. We had enjoyed ourselves. It made sense to make the effort to thank them properly.
The Senior Cat, being a very well brought up kitten and then cat, is meticulous about thanking people - and he means it. My siblings and I have been taught to do the same. 
Many years ago Middle Cat  was part of a national sports team. They were away and won whatever match they were playing. They were celebrating but Middle Cat went quietly around to everyone and obtained a small donation - to thank their coach. They gave some recognition to her as they were leaving the bus for the last time. It still irritates Middle Cat that nobody else thought of it. The Captain of the team should have organised it.
I once took a small group of children to meet an author. We had gone to see him rather than him come to see us. He had a busy job as the head of a teacher training college and he had mobility issues. He spent more than an hour with the group and provided them with very, to them , grown up tea and biscuits. They talked about the visit for weeks afterwards but it was the following day when they said to me, "We have to say thank you again." When I asked which one of them was going to write the letter they looked at one another and then told me they were all going to write individual letters - and they did. He remarked on it to me later - "they really meant it". Yes, you can tell.
As a kitten I was told thanking people, even for help I didn't need, was especially important, "because one day you might really need some help and people won't want to help you". I know the same philosophy applied in a school for children with physical disabilities. I also know that, like me, they learned there are ways to decline help politely but sincerely thank the person offering it.
It's the sincerity which is so important. I had a "thank you" letter recently - except that it wasn't really a thank you letter at all. It was written three months after the events had occurred. It dripped with insincerity. It was perhaps one of the most insulting letters I have ever received. It would have been better if the writers of the letter had said nothing at all. I know that the letter was written under duress but that made it no easier to read and I can't accept the sentiments expressed in it. Far from reassuring me that I had contributed something worthwhile it has raised all sorts of questions in my mind. No, I am not being over-sensitive. I showed another member of the group the letter I had received and her reaction was, if anything, even stronger than mine. "Why," she wanted to know, "did they bother to write that when they obviously don't mean a word of it?"
Why indeed. The only thing that saved me from complete despair over the situation was other people going out of their way to thank me long before the letter was sent. I don't doubt they were genuine.
Thanks must be measured and appropriate and prompt.